I've read a few threads that recommend using Shell Rotella T6 synthetic motor oil. Is this available in something other than 5W40. The reason I am inquiring is because the owners manual does not recognize this as a recommended oil. It seems the manual recommends 10W30 up to 20W50 depending on temperature range. Temperature is a big consideration since I am living in the Arizona desert but it is starting to cool down into the low 100's upper 90's. I guess my biggest question is if you get the best performance from the Shell Rotella T6 even though it isn't a recommended oil?