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By Lefty
FLYER Club Member  FLYER Club Member
#1810112
Multi grade oils are not very successful in engines that mid life - or higher powered engines (250 hp and above).

I didn’t read Mike Busch’s papers on the subject until after I had switched to multigrade- at the cost of 1 burnt out cylinder and and approx 2 - 3 USG per hour in extra fuel burn.

Mine was a mid life 300HP Lycoming when we followed the trend and recommendations and switched to multi. On the flight back to base I immediately noticed that the engine was running much hotter than previously and I had to richen the fuel flow by more than two gallons per hour to keep the temps within range.

After discussion with my engineer we agreed to run it for 2 months / 20 hours to see if it settled down. However at the end of this period we found we had one burnt out cylinder.

It would appear that this was due to:
1. Multigrade doesn’t have quite as good heat dissipation properties as mineral oil
2. Mineral oil holds all the dirt and muck in an engine in suspension, which means that the muck just keeps circulating around the engine. Multigrade doesn’t do this. Instead, it tends to flush the dirt around and the dirt tends to settle in nooks and crannies and block or restrict the flow of oil around the engine - resulting in localised overheating. After replacing the cylinder and filling with mineral oil, the engine has performed for more than 500 hours without problem.

New - or low powered engine - multigrade OK
Older, or high powered engine - mineral rules.
Just my opinion