View Single Post
  #21  
Old 02-03-2006, 09:50 AM
LarryBible
Guest
 
Posts: n/a
Quote:
Originally Posted by Duke2.6
SAE grade is just one aspect of motor oil selection and is dictated by expected ambient temperature cold starts especially on the low end. (See your owner's manual.)

The other more critical selection is API specification, which determines the additive package of detergents, dispersants, anti-wear, anti-corrosion, and other addtives.

Modern oils for spark ignition engines have generally seen a reduction in additive concentration over the years and some of the new SM oils may have NO ZDDP antiwear additive. This is probably okay for modern engines that have fewer sliding surfaces (roller lifters, rockers, etc.) but older engines with flat tappet cams and non-roller rockers (Like M103s) can use more ZDDP.

Oils rated for compression ignition engines (currently CI-4 and CI-4 Plus) have more ZDDP because diesels don't have catalysts. ZDDP and some other additives have been reduced over the years in SI engines oils partially because it is believed that their additive combustion byproducts may degrade catalysts.

Most modern CI-4/CI-4 plus are also rated SL, but can't meet the SM spec because they have too much ZDDP - okay for modern engines, but not for vintage engines.

To paraphase a well known political quote: It's the additive package, stupid!

Duke
A very informative post. Usually folks hash over the same old stuff. The ZDDP additive information is new and helpful, at least for me.

Thanks,
Reply With Quote