Indmar Standard 360 Operator's Manual page 59

MAINTENANCE
Special Note for Boats Operating on Small Ski Lakes
Some operators choose to use their boats exclusively on small
ski lakes where they never get an opportunity to run their engines
for extended amounts of time (15 minutes or more) in the upper
RPM range (above 4000 RPM). Many of these engines are also
exposed to extended periods of idling RPM below 1000.
Boats used in these conditions often have engines that may suffer
from fuel dilution of the motor oil. This condition is commonly
referred to as "making oil" because the oil level on the dipstick
appears to rise over time. Boats that are operated in these severe
conditions will be required to have more frequent oil and filter
changes (every 25 hours instead of every 75 hours).
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