WITH automakers raising the standards for the oil used in their latest cars, it is natural to assume that the newest blends of motor oil — those certified to the highest performance level — will be the best. In general that is true, but owners of older cars are finding it is not always the case.

As engine oil formulas have been revised to cope with the higher loads and operating temperatures of today’s cars, they have also been modified to deal with other conditions. Such was the case with reduction of zinc dialkyldithiophosphate, or ZDDP, an antiwear additive that is known to impair pollution control systems. But the removal of ZDDP is now being blamed for lubrication failures in older cars.

Most lubrication in car engines is of the full-film kind, where moving parts are separated by a thin film of oil. But in a cold-start, for instance, there may be no oil film, allowing direct metal-to-metal contact. Here, antiwear compounds become essential: when friction generates heat, such additives form protective layers to prevent disaster.

Though oil makers claim new formulations exceed the lubrication performance of previous blends, some concerned collectors are turning to oils formulated for diesel engines, which may still contain the higher levels of antiwear additives, or using aftermarket oil additives.