Hey Jeff,
Are you able to determine if the preload system is working correctly? I don't really think the type of oil you are using is the culprit, I tend to think it might be a lack of the fuel/oil mixture getting to the bearings to keep them cool and lubed, can you check the lube line restictor isn't partially blocked or possibly the line under the diffuser housing to the bearings may also have a blockage. Filters etc....Just a few thoughts.

Cheers,
Smithy.