I have ran both. with the 70/140 I have gone through one gear set in 30 000 kms,(might not have been oils fault?) since then I have used 75/90 with no problems. The ford 8.8 has been around for a long time, since the early 80's it has used the same bearings/ gear sets since then in all 31 spline units. All the pre 97 8.8 were using 75/90 and work fine. The reason ford went with 75/140 is so people would only have to change their oil, once every 100 000 kms. Like stated before the front only has 75/90 because it sees way less work so it will last just as long. As long as you use a decent oil, it is up to level, and it is not full of water, it should be fine.

 

One thing to keep in mind is, if you have a F250 or F350 and tow 10,000 lbs, you want 75w-140 for the heavy duty stuff. It is not a typical F150 grocery getter but a completely different truck.

This is what I have been told, anyway - I got Mobil 1 75w-90 for my F250 but then decided to get 75w-140 if I can find it in synth.