I'm planning on changing out the dif fluid in my teen son's truck in the next couple of months. It's a 7.5 inch 3.73 limited slip. Choices:
1) Motorcraft 80W-90 with Motorcraft friction modifier
2) Castrol Limited Slip 80W-90 (assuming it has friction modifer pre-mixed in)
3) Motorcraft 75W-90 Synthetic (do I still need friction modifier?)
4) some other recommendation
We purchased this truck a few thousand miles ago and have no idea when or if the differential fluid was ever changed. Currently has 190k miles. I'd like a high quality synthetic that we may never change again (I say that because it's driven ~5k miles per year and given a normal 30k fluid change interval, we may not have the truck after 30k miles), but only if it makes sense. Does regular Motorcraft+friction modifer make the most sense? Opinions?
1) Motorcraft 80W-90 with Motorcraft friction modifier
2) Castrol Limited Slip 80W-90 (assuming it has friction modifer pre-mixed in)
3) Motorcraft 75W-90 Synthetic (do I still need friction modifier?)
4) some other recommendation
We purchased this truck a few thousand miles ago and have no idea when or if the differential fluid was ever changed. Currently has 190k miles. I'd like a high quality synthetic that we may never change again (I say that because it's driven ~5k miles per year and given a normal 30k fluid change interval, we may not have the truck after 30k miles), but only if it makes sense. Does regular Motorcraft+friction modifer make the most sense? Opinions?