On conventional oils:
Because of the increased amount of long chain polymers added/needed to turn a 10 Weight base stock into a 40 weight stock when warm, 10W40 and 10W50 burns poorly with increased deposits and ash, and often cause considerable build up and sludge even in conventional piston engines.

Many manufactures have also be attempting (and succedding in some cases) of voiding engine warrenties if 10W40 or 10W50 oils are used in newer high reving car engines.

10W40 and 10W50 is only for the Pre-80s GM and ford dinosaur V6 and V8 engines.

And since this subject has been covered countless times, this thread is closed