2008 Scion xD Oil/Filter Change

My 08 Scion xD is due for it's 10,000 mile oil change. Owners manual says 0-20W; however, with my previous (best ever) auto (90 Nissan Axxess-died at 333,000 mi), I changed every 3,000 and always used Castrol 20-50W. We (central Calif) have months that average 90 degree temp and heat waves well over 100 degree weather. I want to use 20-50W but one Toy/Scion rep said I could void the warranty and/or car sensors would not like 20-50W. I had the first oil change done at the dealer, and was told they used 20W (not what the owners' manual calls for!). Can I use 20-50W and not cause any warranty or car problems? I want to change oil myself, but understand this is some kind of paper filter process.