1. What kind of vehicle you have
2004 Toyota Sienna CE (Cheap Edition) 105k miles
2. What your owner's manual says -- not just viscosity, but certifications (look for acronyms like API SM, ILSAC GF-4, etc.) and change intervals as well
0w-20, 5w-20, 5w-30 don't have manual
3. Where you live...