Oil recommend 1961 GMC 305D V6

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ledslinger

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I picked up my 1961 GMC 4WD Suburban from storage last week, changed the oil before driving it 125 miles home. Drained out the straight 30 I put in before driving it to storage. Put Chevron Supreme 10W30 in. When it was my daily driver in the early 80's, I used to run 10W40.

The owner's manual says "Should not be heavier than S.A.E. 20 grade" DG, DM, DS heavy duty engine oil.

This will be driven just for fun, short trips.

I have a good Stewart Warner mechanical oil gage and when I was on the highway running around 60, it had about 58 psi oil pressure with the 10W30 Chevron Supreme. Don't know the RPM but it has 3.92 gears with I think 31.5" tall tires.

Any opinions on the 20 weight oil recommendation?
 
I've said before- my blood cells are shaped like little Mopar stars, but the GMC 305 V6 is without question one of the toughest, neatest, most desirable engines I've ever come across. Congrats on owning one!

That said, I don't know just a whole lot about their quirks, other than having read up on how the cooling system works (huge volume of water is pumped, but even when the T-stat is wide open about half the volume gets bypassed and mixed with cooled water coming back from the rad- which results in a very uniform block temperature). If you ran 10w40 in the 80s, I would really think you could run M1 0w40 today.

Personally, I would not run 20-weight oil in this engine simply because today's 20-weights are low-ZDDP oils and that engine has flat sliding valve lifters. Like Jeep 4.0s and Rover v8s, it CAN get away with lower ZDDP than, say, a muscle car engine because the valve spring pressures are low and its a very mild low-lift cam. But IMO there's no reason to push it when high-ZDDP oils like HDEOs and M1 0w40 are available. The other things you could look into are ordering Rotella T5 semi-syn in a 10w30 grade, or seeing if the boutique manufacturers (Royal Purple, Amsoil, Redline, etc.) make a 20-weight that has plenty of ZDDP and also enough detergency for street/everyday use.

And as a final thing- if you haven't already found it there's a website and discussion board dedicated to the 305 and the V12 "Twin Six" version:

http://www.6066gmcguy.org/
 
When I rebuilt the engine, I noticed the oil return passages from the heads have coolant around them. The cooling system is 33 quarts, certainly designed to do some work. The radiator is not a tube type, and still sound.

Sounds like you have a few infiltrator GMC oval blood cells too. I get a kick out of the V12 that is two of these put together. I have heard of these being installed in cars for kicks.

I have considered running 15W40 HDEO in it. I doubt the valve spring pressures are very high, so I suspect it doesn't need huge ZDDP. Maybe 10W30 Rotella would be good.

I might add a quart of Joe Gibbs 30 break in oil to the 10W30 Chevron for a bit of ZDDP boost.
 
The GMC 305 has colossal bearings for a low-compression gasoline engine. (Actually it has colossal everything for a low-compression gasoline engine.) The bearings for that engine series were the same from the 305 gas all the way up to the 478 diesel. For 150hp out of 305 cubic inches, I can see why a 20-weight oil would work.

But out of respect for the solid flat-tappet valvetrain, I would not rush out and put in an SM/SN Xw20 oil. You could buy an SN Xw20 and supplement it with a zddp additive (ZDDPlus or Red Line Oil Break-In additive). High-zddp 20-weight oils are very rare because that grade is mostly formulated to comply with API specs for Phosphorous content. My usual recommendations for high-zddp gasoline oils are Valvoline VR1 or Brad Penn, but neither of those are available in 20-weight. Red Line 5w20 or 0w20 would be possibilities, if you want to spend the extra money.

By the way, do you post over on the 6066 GMC Guy board? I'm on that site from time to time.
 
I'm not on that board, I have owned this truck since 1978 and have some renewed interest in it. I will likely be joining 6066 and stovebolt forums.
 
Thats the one thing that set the GMC line apart from the "mainstream" Chevrolet line....the old V6 engine.A truck engine,made for trucks,made to last.Low line Chevrolet had to make due with straight sixes...
Now GMC and Chevrolet are carbon copies,although the new GMC nose is far more acceptable than the horrid Chevy one with all that chrome and those two ugly plastic painted bumper ends.
 
Originally Posted By: Loobed


I didn't know that they had a V-6 in '61. I thought they were all straight sixes.

The only thing I could suggest is either 15w-40 or 5w-40 HD Oil.

agreed.
 
Originally Posted By: NHGUY
Thats the one thing that set the GMC line apart from the "mainstream" Chevrolet line....the old V6 engine.A truck engine,made for trucks,made to last.Low line Chevrolet had to make due with straight sixes...


And except for the 292, they were pretty lousy straight sixes too. Even the 292 was only equal to, not better than, the Ford 300 and Mopar slant-6, IMO. I'm not really a fan of the Stovebolts and their descendents, but I do like the 305. Its a beast. For sheer longevity it puts the smallblock Chevy v8 to shame.

I say again... think about M1 0w40. It should prevent overstressing that monster HV oil pump on cold start, plenty of anti-wear additives for the cam, and an overall add pack that rivals HD oils.
 
Many years ago, I worked on a horse farm that had a GMC dump truck. It had the 478 V6 in it. It was the first time I had ever seen a V6 that big. It was reliable as anything I have ever seen. It always started and ran,even in sub zero temps, never overheated and always took you where you needed to go, usually slowly. They ran what ever bulk 10w-30 or 15w-40 oil they had on hand. Usually Delo or Rotella.

The owner of the farm said he bought it used in 1970, and he never had any trouble with it. He said he would clean and adjust the points every few years, and apart from oil changes and coolant changes, he said nothing ever really needed fixing. When I quit there, it had 312,000 miles on it. I still see that truck on the road to this day. Ever since then, I became fans of these big block GMC V6 engines.

Congrats on owning one and keeping it alive!
 
Originally Posted By: A_Harman
The GMC 305 has colossal bearings for a low-compression gasoline engine. (Actually it has colossal everything for a low-compression gasoline engine.) The bearings for that engine series were the same from the 305 gas all the way up to the 478 diesel. For 150hp out of 305 cubic inches, I can see why a 20-weight oil would work.

But out of respect for the solid flat-tappet valvetrain, I would not rush out and put in an SM/SN Xw20 oil. You could buy an SN Xw20 and supplement it with a zddp additive (ZDDPlus or Red Line Oil Break-In additive). High-zddp 20-weight oils are very rare because that grade is mostly formulated to comply with API specs for Phosphorous content. My usual recommendations for high-zddp gasoline oils are Valvoline VR1 or Brad Penn, but neither of those are available in 20-weight. Red Line 5w20 or 0w20 would be possibilities, if you want to spend the extra money.

By the way, do you post over on the 6066 GMC Guy board? I'm on that site from time to time.


This is an interesting idea. Design an engine with a variety of potential power outputs and duty levels, both Diesel and gasoline, and the least powerful version is the one built in the largest quantity. When you think about the heavy duty engines from Ford (360/390) and Chevy (396, 427) they may have been light duty engines first, and modified for heavy duty use.

With today's business logic, the GMC light duty line would have gotten the same inline 6's and 283 V8's as Chevys. I'm sure glad at this time it was important to somebody for GMC to have its own identity.

This engine in 145 HP (305 A) and 165 HP (305 D) appears very undestressed. The crank, rods, and block are very substantial pieces that seem to have potential for very long life.

The strange thing about this motor is that it doesn't have the character of a truck motor. It is way oversquare with a 4.25" bore and a 3.5" stroke. It isn't a low end torque stump puller like truck motors are reputed. It likes to rev and hums along happily at highway speeds.

The bearings are big and offer lots of area for a low stressed engine. It would be interesting to see if the viscosity recommendations get heavier in the higher horsepower versions of this engine. I also wonder if owners were compliant or reluctant in abiding by the 20 weight oil spec.

I had an owner's manual for this truck since I bought it in 1978. I thought the 20 weight spec strange. My impression was 20 weight was what you ran in the winter before multi viscosity oil came out. I suppose I just decided 10W40 was a universal oil back then. It didn't kill it.

Even now I and most response are hesitant to use the recommended oil, but this hesitance is partly due to zddp levels. There are no high zddp level 5W20 oils since the first ones were SJ oils, and zddp levels have been trending down since. The perfect oil for this engine may be a 10W20 with minimal or no viscosity improvers, and over 1000 ppm of zinc. The closest thing would be a straight 20W HDEO.

Low oil temps could be a factor in the 20 weight spec, as well as the large bearing area. With 33 quarts of coolant and capacity for greater horsepower, the oil temps could be low. I'll have to do some testing on that. I may be looking for 20 weight HDEO if the oil temps hover around 160 F.
 
ledslinger- Don't forget that the famous (or infamous depending on who you talk to) Toro-Flow diesel was based on the V6 architecture.
 
If you want to see how massive these engines are built, here is a jounal of a student who is making performance parts for a GMC V6:

http://www.hotrodders.com/forum/journal.php?journalid=75212&action=view

You can see photos of a V6 rod and piston next to one from a 350 small block for scale. There is also pics of his attempt to build a performance intake from sheet metal for a V6. Not fantastic welding skills but neat design. I'm sure he learned a bunch.
 
Originally Posted By: ledslinger



The strange thing about this motor is that it doesn't have the character of a truck motor. It is way oversquare with a 4.25" bore and a 3.5" stroke. It isn't a low end torque stump puller like truck motors are reputed. It likes to rev and hums along happily at highway speeds.


I can think of a number of oversquare gasoline truck motors from that era- in fact ALL of the Chrysler "-3" heavy-duty gasoline engines were oversquare to one degree or another with the sole exception of the the 225-3 slant-six. The 318-3, 361-3, and 413-3 (as well as the later heavy truck/motorhome versions of the 383 and 440) were all well into the oversquare zone, and the 361 big-block was more oversquare than the later 360 smallblock. I think its actually a mis-conception that undersquare is necessarily more "truck-like." Oversquare engines with large bearing area can actually produce more torque than a theoretical engine of the same displacement built under-square, because torque goes up with the SQUARE of piston area, but only goes up linearly with stroke if you assume the same BMEP. So as you trade stroke for bore, the available torque goes up, PROVIDED that the bearings can take the far higher forces acting on them due to the large piston area. The real death of extremely oversquare engines came with emission regulations since its harder to get a clean burn with a very large bore.

Quote:
Low oil temps could be a factor in the 20 weight spec, as well as the large bearing area. With 33 quarts of coolant and capacity for greater horsepower, the oil temps could be low. I'll have to do some testing on that. I may be looking for 20 weight HDEO if the oil temps hover around 160 F.


You're probably right about that engine having low oil temps. But the other reason, frankly, that I don't feel too bound by the 20-wt spec is because its from 1960 when oils with high VI were unheard of. They were probably trying to maximize cold flow and even compromising a little bit on viscosity at operating temp knowing that the sheer butch-ness of the engine would protect it. You can get superb start-up cold flow today with a 0w40 or 5w40, and still not be too thick even at a relatively low operating oil temperature. The oil pressure gauge will tell the story for you.
 
A Harmon- What a project! How's it going? That will be a cool and unique truck when you are done. What is the recommended oil for the 478?

I looked at your pics, could you explain the cut cam? Did you have to rob the dist drive gear and install on the new cam? Is the last cam journal bolted on?
 
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