The lube oil quality management program used on your offshore supply vessel requires the on-board testing of lubricating oil viscosity at consistent temperatures. An unusual drop in lube oil viscosity indicates excessive dilution of the lubricating oil with diesel fuel oil. What fuel dilution percentage would be considered the maximum allowable and anything above warrants an oil change after repairs are made?
• Lubricating oil viscosity and film strength in diesel engines • How diesel fuel dilution affects lubricating oil properties and bearing protection • Typical manufacturer/industry limits for acceptable fuel dilution before an oil change is required
• Think about what happens to the oil film on bearings and cylinder liners when a light, low‑viscosity fuel like diesel mixes with heavier lube oil. At roughly what percentage would you expect that protective film to be seriously weakened? • Compare each percentage choice with what would still be considered ‘normal’ trace contamination versus ‘dangerous’ contamination in a working engine. • Which of the options looks more like an early, conservative maintenance limit versus a level where the engine would likely already be at high risk of damage?
• Eliminate any percentage that seems so high it would almost turn the oil into a fuel mix rather than a lubricant. • Consider that the maximum allowable limit is usually set before serious damage occurs, so it is not an extreme, catastrophic level. • Ask yourself which value aligns with common diesel engine oil analysis reports, where labs flag fuel dilution first as a caution, then as ‘change oil now’.
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