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Engine Oil spec

Engine Oil spec

Engine Oil spec

Is there more to specifying engine oil than just the viscosity?

I want to use a certain grade in my petrol (gasoline) car (15W-40). The container specs its for Diesel use, but my car book says this grade is fine.

Could there be other additives (or lack of) that make it unsuitable?



"Tell a man there are 300 billion stars in the universe and he'll believe you. Tell him a bench has wet paint on it and he'll have to touch to be sure."

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RE: Engine Oil spec

In a word, yes.  However if the oil is suitable for a gas engine, it should have an S(X) rating (spark ignition) in addition to the C(X) rating (compression ignition).  As long as it matches your oil requirements, say SH or SJ, etc. its rated for use in your engine.


RE: Engine Oil spec

Also be weary of detergent additives in Diesel oils. Most modern 'clean' engines can handle them but I would avoid them in older models with high-mileage engines. Most of these engines require the years of build up on seals and bearings to actually remain 'tight'...the detergents in Diesel oils can loosen this build up, much like oil additives to 'clean' engines...the worst thing you can do to an old engine.

RE: Engine Oil spec

I am not an oil expert by any stretch of the imagination but, in my 63 years, 50 of them building cars, I have always found the concept of the "detergent"  in lubricating oil having the ability to suddenly and totally clean and release years of built up varnish and sludge as rather amusing.  Replace the word 'detergent' with dispersant' and your on the right track.  I restore old cars, very old cars with very old engines, some as old as 73 years with engines in original condition.  I have NO qualms about using modern oils and have thus far experienced no problems with the dreaded 'false' seal failures everyone is so concerned about.
Using an S(x) rated oil in a CI engine could be a problem (perhaps not enough dispersants for CI use) but, using a    C(x) rated oil in an SI engine should not raise undue concern in normal use.

That's about $.02 worth.


RE: Engine Oil spec

I too have restored many an old engine, and whilst I agree, that in most cases, any form of 'dispersent' will not "suddenly and totally clean and release years of built up varnish and sludge", I have seen it to be the precursor of many troubles.

The Diesel specific 'detergents' are there to eliminate and/or reduce the amount of soot build-up as a by-product of Diesel Combustion and oxidation. A process which obviously does not occur (as predominately) in petrol engines. The detergents in Diesel oils also 'scavenge' the free carbon-12 particles in lower than optimal conditions, as opposed to Nitrous Oxide absorption, another purpose of the 'detergents' in Diesels. Due to the lower temps and pressures in petrol engines this leads to a steady reduction in the hardness of existing carbon compounds found in oil seals.
I am not saying that this will occur in EVERY engine, and to avoid Diesel oils like the plague...a nice, thick 40/70 Diesel Oil can do wonders for an 'ole smokey petrol engine, but I have also seen it destroy what little seals one also had left in it. It depends on varying factors regarding the seal types, age, oil type previously used, oil/filter change schedule, correct timings, and general condition of the motor.

I just cant see why one would put a 15W/40 Diesel Oil in a petrol motor. If they were one and the same, they wouldn't sell seperate Oils for Petrol cars.

That's just my opinion and it's probably worth less than evelrods ($0.01?)

RE: Engine Oil spec

I went looking last night; Rotella T 15w-40 does not have any spark ignition (Sx) ratings, Delo 15w-40 does.

Anyone got change for a penny?


RE: Engine Oil spec

I know this is avery late reply,having just joined up with this forum. Modern petrol/diesel engine oil essentially share the same additive system packages.,just used in slightly different proportions. API SJ lubricants have similar requirements for engine cleanlinless as does CH4 diesel emgines. Any older engine say from 80,s onward that has had regular servicing can safely use newer petrol oil that are branded for diesel also eg SJ/CF. I woulndnt go the other way for older vehicles  15w/40 CH4 mack /sj,as they are designed for truck long drain intervals and have a high TBN

RE: Engine Oil spec

Diesel lubricant will work fine if you DON’T have a catalytic converter, engine seals that cant be damaged by high amounts of sulfur and you DO NOT want to get decent fuel mileage.

RE: Engine Oil spec

The current crop of synthetic "0W-20" oils may be great in satisfying OEM CAFE but LESS pressure on the old 9D's will get you better fuel mileage---cheaper, too!

BTW---This whole line of conversation is only worth that two cents if the car/engine in question is not already in "failure mode".  If it is, no motor oil will fix it, or make it last longer, or give you better fuel mileage, or...
However, if it burns a lot of oil it will most certainly kill ANY catt.


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