This thought popped into my head on the way to work today. If you used a different weight oil that was "required" by your manual and something went wrong in the engine and the dealership blamed it on you using the wrong oil (but it is an oil that is used in other countries in the same motor. ie: my '93 3VZ-E is spec'ed here for 10w30 (5w30 for much colder temps)only, but I think a 20w50 is stated suitable for use in Australia in high temps in the same engine), could you use the Aussie specs to show the dealer they are full of garbage...and make them pay up? I don't see how any car manufacturer that sells cars overseas with different oil weight specs than ones sold here can void a warranty due to this fact. The oil would have to be one stated "OK for use" in a foreign manual, remember.