: Diesel oil in gasoline engines...BBob, opinion?

04-10-04, 10:07 PM
Some guys on another forum I participate in were extolling the virtues of running diesel oil in place of regular gasoline oil, in gasoline burning engines. They cited extended oil change intervals and higher quality.

What are your thoughts on this?

04-10-04, 10:21 PM
Not to butt in anywhere... but as long as we're talking crankcase oil... They're right. Diesel engines run at many, MANY times the compression ratios of our engines, and hence take on lots of cylinder pressure, and load on the bearings. So if you see any oils that are rated for diesel engines, they're more than adequate for gasoline engines... Although I think most already are rated for both.

El Dobro
04-11-04, 12:11 AM
Diesel oils have different ratings than oils for gas engines. They're usually heavier weights than GM recommends for gas engines and I wouldn't doubt that the additives are totally different since they're working with different fuels. Personally, I wouldn't mess with it.

04-11-04, 12:30 AM
Next time you buy oil... check for the API star on the back (says the weight and some letters on it). S means gasoline engine (I think the latest and greatest is SL. C is for commercial engines (diesel), their latest is CD I think... Diesel oil has more sulfur content in it among other things... See if you can find an oil with both S and C ratings on it. Those are typically really good, and then you at least know it's safe for your engine.

04-11-04, 11:11 AM
The "diesel" oils that they are talking about are probably the Delo, Delvac and Rotella oils.... those are excellent oils and perfectly good for diesel as well as gasoline engines.

If you look at the performance ratings on the oils there are two that are important : the SL rating and the GF3 or GF4 rating.

Fundamentally, the "GF" ratings are intended for gasoline engines and were instituted to remove confusion as to what oil should be used in gasoline passenger car engines. While this has had the desired effect it has created a bit more confusion in the long run. While the GF3 and GF4 ratings include the same performance requirements for the SL ratings they ALSO include some minimum friction performance standards. In the interest of fuel economy in gasoline engines friction reducers are added to the oils. The GF ratings commonize these friction reduction standards so that the automakers can test on the FTP with the improved fuel economy oils. Gasoline engines are fine with these standards....modern gasoline engines, anyway.

The heavy duty oils such as the Delo and Delvac oils are rated SL for performance....but do not include the friction reducers so they are not "GF" rated oils. This is good for older gasoline engines and for vehicles such as motorcycles that have wet clutches that react poorly to friction modifiers in the oil.

In the past there were heavy duty diesel ratings such as CC and CD for normally aspirated and turbocharged diesels. The oils were fortified for more wear protection and good high temp deposit formation resistance. These CC and CD standards have pretty much been superceded by the superior performance standards of the SL classification. If you read the can those standards are referenced along with the SL requirements.

I generally recommend that people with older muscle cars use the Delo, Delvac and/or Rotella oils. Those engines have flat tappets, distributor gears, spur gear oil pumps, rocker arms with sliding style pivots, etc...that need more lubrication and more of the antiwear compound (the ZDP) that is found in the heavy duty oils. Modern gasoline engines, however, are better off with a GF rated oil. Modern engines have rolling element pivots/tappets, no distributor gears, gerotor style oil pumps, etc. and are relatively insensitive to the ZDP level and do not deplete the oil nearly as fast...and they can utilize the lower friciton oils for better fuel economy.

I would use the oil that was recommended for the engine. There is really nothing to be gained in a modern Northstar or other engine by using the Delo, Delvac or Rotella oils. Just no need for them. And the fuel economy in day to day use will suffer if a GF rated oil is not used.

Motorcycles and muscle car engines and the older of the 4.1/4.5 engines can benefit from the heavy duty oils and don't really need nor want the friction modified oils.

The main thing to understand is that there is no single "correct" answer for which oil to use. Anyone that says that a given oil is the "best" doesn't understand oil or engines. Modern gas engines are designed for the GF3 rated oils and can use them and maintain the longer change intervals specified according to the oil life monitor with no problem. Older engines that are still running around will probably live ok with these oils but can benefit from the added protection of the heavy duty oils such as the Delo, Delvac or Rotella products.

As far as change intervals, there is no rational to support the idea that the Delo, Delvac , Rotella or other heavy duty oils can be used for longer drain intervals than any other oil. Refer to my post on the "synthetic lubes" and other posts where the reasons for oil changes are outlined. Several of the constraints for oil change intervals have nothing to do with the type of oil...such as contamination, cold starts, oil oxidation, etc.... Use the recommended change intervals, or, better yet, follow the oil life monitor recommendations on the GM cars rather than trying to second guess the guess that designed the engine by substituting different types of oils, etc....

Sinister Angel
04-11-04, 11:28 AM
Real men run Straight 30 weight Mobile oils :P

When I saw the title, I was thinking they were talking about diesel fuel oil and I was like "wtf?!?"

04-11-04, 02:12 PM
...^^ Took me to school! :worship:

04-14-04, 10:41 PM
Thanks for the responses, everybody!

04-14-04, 10:52 PM
There are 2 other considerations.

1) Your warrenty may not cover an extended oil change, even if the oil is better.

2) The oil filter may not support the extended length of time.

IMHO, Do as the Operator Manual states.