Candid Cameron

CANDID CAMERON

As a follow-up to the Candid Cameron column that appeared in the January, 2005, issue, I would like to ask: In contrast with the engine-oil recommendations found in car owner’s manuals, which state a minimum API service category requirement and allow any equal or higher category, the motorcycle manufacturers (the Japanese, at least) specify specific and quite old categories. Usually, it is SF or SG, but some (e.g. Yamaha for the R1 and R6) go down even to SE (which became obsolete in 1979 and is unavailable). Is it permissible to use oils of a higher API category than specified, and if not, why? The API declares that any newer engine-oil service category includes the performance properties of each earlier category.

I’m aware that newer categories may contain lower concentrations of some additives, but as far as I know, this concentration reduction is obligatory only to the lighter oils (e.g. SAE xW-30, where “x” can be 0, 5 or 10), which are not used in motorcycles. What’s more, the API specifications limit the maximum quantity of these additives; so in any case, without a chemical oil analysis, we can’t know what are the actual additive concentrations in any specific oil, regardless of its API classification. According to comparative tests appearing occasionally on the Internet, oils conforming to older categories are not always better in the additive concentration respect than oils bearing newer ones. Could you shed light on this issue? Uor Gal

As you stated, I touched on this topic 10 months ago, but I apparently need to elaborate. API “S” oil categories are for spark-ignition engines, the majority of which are in cars. Therefore, API categories are primarily createdfor the auto side of things, and autos have catalytic converters that can be poisoned by certain metals. The anti-wear additive zinc dithio dialkyl phosphate (ZDDP) is important to such things as cam-to-tappet wear and piston-ring wear near TDC, for it forms a surface layer that can support heavy loads as a solid lubricant, protecting metals from damage when the desirable full-film “hydrodynamic ” support by the oil breaks down under heavy pressure or low sliding velocity. Think of hydrodynamic support as being the “surfing ” of a moving part on a viscous oil film. Think of support by an additive as being like the powdered graphite people blow into a sticky lock to make it work freely.

When an auto engine ages, it passes more oil into the combustion chamber and through it to the exhaust and the catalytic muffler. The ZDDP anti-wear additive gives up its zinc at high temperature, and the zinc “poisons ” the catalyst, making it ineffective. To prevent this, more-recent oil standards have called for reduced ZDDP. To restore its function, so-called “friction modifiers ” such as oleic acid are added to the oil in place of the ZDDP. Friction modifiers cause devices such as sprag-type motorcycle starter clutches to slip, and may also cause the driveline clutch to slip. It is for this reason that motorcycle manufacturers call for use of older API standards containing more ZDDP and less friction modifier.

Yoqne’am lllit, Israel

Kevin Cameron