I have a 99 dodge cummins with the 24 valve motor. The owners manual says to use 10w-30 from 0 to 30 degrees f. or 5w-30 syn. from -10 to 20. I found rotella t syn. 5w-40 and was wondering if I could use that? Whats the difference between 5 30 and 5 40? I would like an oil that will work down to 20 or 30 below to 40 above. Any suggestions? Thanx