10W30 is perfectly fine.
Ran it year round for a few years. The 5W weight is primarily for them to claim higher gas mileage and for quicker starts (oil flow) in cold weather but if you look at the pour points there is not much different between a 5W and 10W in pour points.
I also do not believe the 5W20 protects as well as a ?W30. Again, they need to up the MPG and a lighter weight will do that, they don't care about long term wear after the warranty
