Rear Diff oil for 2005 F150

[BrianA]

This is also posted in the Oil and Lube forum, but would like input from here as well. Thanks in advance for any help. --------------------------------------------- 2005 F150 Not LS rear end I have to replace a leaking axle seal, so I get to open the diff and must therefore replace the fluid. Ford calls for 75W-140 fully synthetic. Local shop has one bottle, but several of same gear oil in Valvoline conventional (not synthetic) QUESTION: Is synthetic necessary for my application or does Ford spec it because it could be considered "lifetime" and no drain/replace interval needed? Thanks in advance for any help. edit to add: I just looked back (online) at some mid-late 90's model F150 specs. Those call for 80W90 conventional. I don't think the rear end has changed, so that really makes me wonder if the 75W-140 is required. What should I do? Go with the synthetic? Use 75W-140 conventional? Use 80W90 conventional? Thanks!

[aurgathor]

Strictly speaking, 75w140 is not required, but it's a better oil; furthermore, if you have any warranty issues, 80w90 in there may be a sufficient reason to void the warranty on the rear.

The only scenario in which I'd use 80w90 if the axle were submerged frequently, necessitating frequent oil changes.

[BrianA]

Thanks for the response, and hope for more.

For the record, it has 112,000 miles on it right now, so warranty is long since gone.

[aurgathor]

In that case, there's no need for 75w140. My factory manual said not to mix synthetic and regular gear oil, so you want to drain as much as possible before putting in a different oil.

[rsylvstr]

I'm using 75W-140 Amsoil and suggest it for your application also....since it's only 2 qts, it's about $15 and worth it.