What defines straight mineral oil used in aircraft lubrication?

• A. It is the lubricating oil obtained by fractional distillation of crude oil with no additives.
• B. It is the lubricating oil that contains ash-forming additives.
• C. It is synthetic oil produced by chemical modification.
• D. It is automotive oil designed for road engines.

Answer: (B)

Straight mineral oil in aircraft lubrication is defined by containing ash-forming additives. These additives introduce metals such as zinc and calcium that form ash when the oil is burned or exposed to high temperatures, providing film strength and anti-wear protection in certain engines. This distinguishes it from oils designed to be ashless, which are formulated specifically to minimize residue. It is not synthetic oil, which is produced by chemical modification of base stocks, nor automotive oil intended for road engines rather than aircraft use.