Mercedes IDI 30 wt. vs. 40 wt.

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1,233
Location
Campbellsville, KY
My wife and I both drive Mercedes W123's with the OM617 turbodiesel. This is a 3.0L IDI 5-cylinder mill breathing through a Garrett T3 turbo that holds 2 gallons of oil and has an air>oil cooler. On all the enthusiast forums they preach nothing but 40 weight - but there are also a lot of people that get nasty when you mention synthetic. "The synthetic is too thin for these old iron motors. They weren't designed for it. Go by the owner's manual." Well... the manual says this: 32F and up: SAE 30/20w40/20w50 23F and up: 15w40/15w50 -4 to 50F: 10w/10w30 All climates: 10w40/10w50 Both engines get good fuel economy, use no negligible oil, and get Schaeffer's 9000 5w40 changed every 5,000 miles. I don't want to risk the motors on a casual experiment, but will using a synthetic 10w30 (to the chagrin of the old-hat enthusiast crowd) run any risk of compromising the motors through the cooler months here in KY? My gut says no, the modern 30 weight will be in better shape after twice the miles that a 40 weight would've been in the 1980's - but I won't discount the opinion of people with more experience than myself just on a whim.
 
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14,387
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Upper Midwest
What Mercedes Benz approval is required for your engine? That dictates a minimum HTHS and subsequently the grade. Is there no approval for an engine that old? People that preach grade when it is divorced from a required approval don't know what's important. Is your gut feeling an adequate replacement for approvals?
 
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1,233
Location
Campbellsville, KY
I'm not finding any correlation of a MB spec. to the cars of this era - just the API ratings, at best. The 1980 owner's manual I have handy says "consult your Mercedes service center" or something along those lines, like the oil spec may change - which wouldn't be a bad move considering the evolving nature of motor oils and the possibility to back-spec a more modern oil after the cars leave the factory.
 
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1,233
Location
Campbellsville, KY
In a nutshell, what would be the benefits over the synthetic 5w40 I'm using now? These engines run forever on... basically anything. The majority of people just default to T4, Delo, etc. 15w40. Usually what kills them is being run low/out of oil, or just succumbing to decades of only ~adequate~ maintenance.
 

Astro14

Staff member
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Virginia Beach
As a former W123 owner, I remember the wide viscosity range allowed in the owners manual. SF/CC was the spec listed I think. Hardly rigorous. Plenty of latitude in that manual. If I still had a W123, the Rotella T6 synthetic (5W40, inexpensive, good slot control) would be my ‘year round choice. A 10W30 in KY winters would be fine, too. It doesn't need a boutique oil, it's a tolerant engine. But good detergent/soot control are important.
 
Messages
1,233
Location
Campbellsville, KY
Originally Posted by Astro14
As a former W123 owner, I remember the wide viscosity range allowed in the owners manual. SF/CC was the spec listed I think. Hardly rigorous. Plenty of latitude in that manual. If I still had a W123, the Rotella T6 synthetic (5W40, inexpensive, good slot control) would be my ‘year round choice. A 10W30 in KY winters would be fine, too. It doesn't need a boutique oil, it's a tolerant engine. But good detergent/soot control are important.
Slot control? Yes, the IDI's dirty the oil more quickly than others so keeping that under control is the biggest concern. With a tight, healthy engine with good oil pressure and no problems with a thinner oil slipping through the cracks (rings or gaskets) I don't see why a 10/30 wouldn't protect one just as well.
 
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10,418
Location
Cincinnati, OH, USA
I cleaned up a coked up ‘82 300D turbo engine with Delvac 1 5W40 when I first bought it from an older guy that ran gasoline engine 20W50 in it (!), it did great, but it leaked it out pretty quickly. Any decent diesel rated HDEO would do great in those dirty old diesels, from 5W30 on up to 15W40.
 

Astro14

Staff member
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11,069
Location
Virginia Beach
Originally Posted by Alex_V
Originally Posted by Astro14
As a former W123 owner, I remember the wide viscosity range allowed in the owners manual. SF/CC was the spec listed I think. Hardly rigorous. Plenty of latitude in that manual. If I still had a W123, the Rotella T6 synthetic (5W40, inexpensive, good slot control) would be my ‘year round choice. A 10W30 in KY winters would be fine, too. It doesn't need a boutique oil, it's a tolerant engine. But good detergent/soot control are important.
Slot control? Yes, the IDI's dirty the oil more quickly than others so keeping that under control is the biggest concern. With a tight, healthy engine with good oil pressure and no problems with a thinner oil slipping through the cracks (rings or gaskets) I don't see why a 10/30 wouldn't protect one just as well.
Apple "correcting me" again. Soot. Variously corrected to "slot" or "shoot". Those engines make quite a bit of carbon that dirties up the oil. Good detergent is a must. (any diesel-rated oil will do). They also pre-date MB 229.3 or 229.5, so while those are good specs, they're not really a concern. Just get a modern diesel-rated oil. 10W30 Rotella, or Delvac, or Valvoline, or whatever, as long as it is diesel rated, would be fine.
 
Last edited:
Messages
10,418
Location
Cincinnati, OH, USA
Originally Posted by Astro14
Originally Posted by Alex_V
Originally Posted by Astro14
As a former W123 owner, I remember the wide viscosity range allowed in the owners manual. SF/CC was the spec listed I think. Hardly rigorous. Plenty of latitude in that manual. If I still had a W123, the Rotella T6 synthetic (5W40, inexpensive, good slot control) would be my ‘year round choice. A 10W30 in KY winters would be fine, too. It doesn't need a boutique oil, it's a tolerant engine. But good detergent/soot control are important.
Slot control? Yes, the IDI's dirty the oil more quickly than others so keeping that under control is the biggest concern. With a tight, healthy engine with good oil pressure and no problems with a thinner oil slipping through the cracks (rings or gaskets) I don't see why a 10/30 wouldn't protect one just as well.
Apple "correcting me" again. Soot. Variously corrected to "slot" or "shoot". Those engines make quite a bit of carbon that dirties up the oil. Good detergent is a must. (any diesel-rated oil will do). They also pre-date MB 229.3 or 229.5, so while those are good specs, they're not really a concern. Just get a modern diesel-rated oil. 10W30 Rotella, or Delvac, or Valvoline, or whatever, as long as it is diesel rated, would be fine.
Yes, DO NOT use a dual rated passenger car oil (like SF/CC, CD, or CF) like some of the manuals suggested-that's what jammed up the rings on my old 300D, it couldn't handle the soot buildup. After the Delvac 1, I ran whatever cheap 15W40 (summer) or 10W30 (winter) CI-4/+, CJ-4, or CK-4 oil I could find. Mostly Delvac 1300 & Rotella Triple Protection & T5.
 
Messages
61
Location
TX
Originally Posted by bullwinkle
I cleaned up a coked up ‘82 300D turbo engine with Delvac 1 5W40 when I first bought it from an older guy that ran gasoline engine 20W50 in it (!), it did great, but it leaked it out pretty quickly. Any decent diesel rated HDEO would do great in those dirty old diesels, from 5W30 on up to 15W40.
You ran only one oci of delvac 1 and it cleaned it up that drastically? If that's true that's pretty impressive. What were the indications it was coked up?
 
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