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#90764 - 07/10/04 01:53 AM Gear Oil in engine oil
metroplex Offline

Registered: 09/12/02
Posts: 1925
Loc: SE MI
Has this been discussed before?

If not:

What would happen if I added 1/4 - 1/2 qt (maybe 1qt if I'm in the experimenting mood) of 80W-90 gear oil to 4-5 qt of oil (say 10W-40).

I have a 1981 Ford 302 V8 that loves thick oils. If you use xW-30, the gas from the carb will promptly thin it out in no time causing it to flood the combustion chamber (weak rings? I installed new valve stem seals at 95k miles).

If you use 10W-40, it runs great.

Now I've been wondering - would the additives in gear oil work great in engine oil coupled with actual engine oil? Gear oil generally isn't cooled by anything other than ambient air temperatures. It doesn't get filtered and doesn't get changed for 100k miles. That's quite some abuse for all that metal to metal contact...

#90765 - 07/10/04 02:00 AM Re: Gear Oil in engine oil
Gene K Offline

Registered: 01/25/03
Posts: 3558
Loc: Decatur AL USA
The ratings used for gear oil isnt the same as engine oils a 90 weight gear oil is not the same viscosity as a 90 weight motor oil.

I would use Mobil Delvac 15W-40 for what you are trying to accomplish ($5.88 Gal @ WalMart) and it meets 10W Cold Pump Standards for cooler weather.

Try it, I think it will make a big difference in your application.


#90766 - 07/10/04 04:01 AM Re: Gear Oil in engine oil
Gary Allan Offline

Registered: 09/28/02
Posts: 39806
Loc: Pottstown, PA
Now my knowledge is dated on this ...but if you mix gear oil with motor oil'll get spontanious foaming.

yes, 90 weight hypoid is about a 40 weight.

Try it in a beaker first. I think you'll reconsider.

#90767 - 07/10/04 04:16 AM Re: Gear Oil in engine oil
RB Shannon Offline

Registered: 05/29/02
Posts: 277
Loc: Greencastle Pa.
Anther point is a gear oil has an acidic (as far as I know) additive package and wouldn't mix well with the alkaline adds in motor oil which are designed to nuetralize the acids developed by combustion, the EP additives are far more aggresive than the antiwear package and would probably be activated by the higher oil temps and cause corrosion of the softer metels.
If you just want to thicken it up a bit try something like Schaeffer's 132.

[ July 10, 2004, 07:33 PM: Message edited by: RB Shannon ]

#90768 - 07/10/04 05:13 AM Re: Gear Oil in engine oil
Gary Allan Offline

Registered: 09/28/02
Posts: 39806
Loc: Pottstown, PA
Anther point is a gear oil has an acidic (as far as I know) additive package and wouldn't mix well with the alkaline adds in motor oil which are designed to nuetralize the acids developed by combustion,
Perhaps the phosphorus (spl?) is what reacts and causes the foamation that I have observed. This would make sense.

Anyone have the "fat" on this???

#90769 - 07/10/04 07:03 AM Re: Gear Oil in engine oil
TallPaul Offline

Registered: 10/22/03
Posts: 13101
Loc: By Detroit
My understanding is that gear oil has additives that can and likely will ruin your bearings.

#90770 - 07/10/04 07:03 AM Re: Gear Oil in engine oil
n8wvi Offline

Registered: 09/16/02
Posts: 622
Loc: 42.4N 85.7W
As far as recommendations, I think Gene hit the nail on this one. A HDEO 15w-40, given that metroplex said a 10w-40 works well. For a vehicle in Michigan, I think the Delvac 1300 is a good recommendation.


#90771 - 07/10/04 09:43 AM Re: Gear Oil in engine oil
KJA426 Offline

Registered: 07/09/04
Posts: 651
Loc: Punxsutawney, PA.
You also have SAE 30 and 40 along with 20w-50 for warm temps. The 15w-40 is a good choice.

#90772 - 07/10/04 02:07 PM Re: Gear Oil in engine oil
Ed Offline

Registered: 08/15/02
Posts: 135
Loc: Southern California
Back in the good old days it was common practice to fill the crankcase of a worn out engine with 90w gear oil. The engines would run quietly and not smoke just long enough to sell the car to some poor sucker. Gear oil additives are not suitable for use in engines. It would make a lot more sense to use a product specifically designed to accomplish what you are trying to do. Things like STP, Motor Honey or any of the similar products that thicken the oil really do work,just not for very long. There are much better odds against engine damage with those. A couple of cans of STP just before putting on the "For Sale" sign might result in a quicker sale. Let your conscience be your guide!

#90773 - 07/10/04 04:49 PM Re: Gear Oil in engine oil
widman Offline

Registered: 05/27/02
Posts: 3245
Loc: Bolivia
In the old days (and usually today), the straight weight SAE 90 was/is a GL1. So all you were doing was adding a non-detergent SAE 50 (could be anywhere from a high visc. 40 to a low visc 60) to thicken it up. Some of the straight 90's today have an anti-oxidant also.
Putting a GL-5 oil in the engine will cause problems with additives, since:
1: There is copper and brass in the bearings of the engine where you don't want phos/sulfur.
2: The acidic gear oil mixed with the basic/alkaline motor oil, in the presence of humitity from combustion will cause additive precipitation.
If you want to thicken your oil, just move up in the viscosity table.

#90774 - 07/10/04 05:40 PM Re: Gear Oil in engine oil
Amkeer Offline

Registered: 05/06/03
Posts: 2426
Loc: Florida
Yup, you will fry the bearings adding gear oil to the crankcase!

Daily Drives:
-2003 Toyota Tacoma PreRunner XtraCab, 2.7 Liter , Mobil1 Synthetic SS 5W-30.
ODO 9300Miles.
-1995 Toyota 4-Runner 3.0 V6, Mobil1 Synthetic SS 10W-30.
ODO 91400 Miles.

#90775 - 07/11/04 05:18 AM Re: Gear Oil in engine oil
LarryM Offline

Registered: 09/17/02
Posts: 25
Loc: Oviedo, Florida

For what it's worth, I had a 302 with the 2-barrel carb in a 1969 Fairlane. That carb had a rubber diaphram in the base of the carb that was supposed to sense intake manifold vacuum. When that diaphram tore, it dumped raw gas into the intake manifold - especially since it got a little help from the vacuum.

I realize that this was quite a few years earlier, but the carb design might not have changed.

#90776 - 07/12/04 03:01 AM Re: Gear Oil in engine oil
MolaKule Offline

Registered: 06/05/02
Posts: 17755
Loc: Iowegia - USA
This has been discussed elsewhere and it is a bad idea.

Gear oil additives and engine oil addditves are specific to the application. They are not interchangeable. In addition, gear lubes and engine oils have different viscosity indexes and different types of VII adds.

Gear oil additives are designed primarily for EP protection and are designed to operate at lower temperatures than seen in IC engines.

Widman has the right idea. Simply move up in viscosity if you're wanting thicker oil.