Which fuel percentage value is considered Unacceptable in the lube oil classification?

• A. 5.0% and greater
• B. 4.9%
• C. 3.4%
• D. 2.0%

Answer: (A)

Fuel dilution in lube oil directly affects the oil’s ability to lubricate. When five percent or more of the oil’s mass is fuel, the oil becomes too thin to maintain a protective lubricating film under engine load. That weakened film leads to increased metal-to-metal wear, higher bearing and component temperatures, and accelerated oil breakdown. Because of this loss of lubrication, that level is deemed unacceptable.

Lower levels of fuel in the oil are generally within acceptable limits, so numbers below five percent are typically considered okay (within the specific oil specification and maintenance guidelines). If you ever see fuel contamination reaching five percent or more, it’s a clear signal to change the oil and investigate the source of fuel ingress to prevent repeat occurrences.