Originally Posted by Speedy

Well, my point was that manufacturers spec an oil, in part, to help with their EPA mileage requirements. You can find this as fact by looking at an oil spec for some cars in the USA vs. Europe. Same car, same engine, and completely different oil specs.

The 5W30 for the Z may be a result of an EPA mileage requirement when in actuality a 5W40 or 10W40 would actually work better for the motor.

I have no information to support that theory as of yet, just throwing it out there.