I've been running 15w-40 in my VP 4.3gxi for a few years now. I do one oil change a year as my yearly hours remain low. I noticed Wal-Mart is carrying supertech 25w-40 now which I assume is just rebadged Quicksilver. Should I continue with 15w-40 or switch? I'm running 15w-40 dino currently, but thought if anything, going to Rotella T6 full synthetic. What say you?