LowNSlow,
There could be a small problem with multigrade oils not being approved for older out of production engines.The manufacture would usally approves a new type of oil for use with its engine after testing or making sure that it meets the required spec's MIL/SAE etc. by some other way.If its not approved then it can be regarded as using a non approved part with all the safty and legal problems that brings.Or the oil may just not work with the materials used in that engine.Check your POH/AMM.You could try the oil manufactures to.
You could try doing your oil/filter changes earlier and using a good quality oil.But warm up the engine before any high RPM.That is one of the best ways of reducing ware and tear on any engine.
