i put 75-90 mobil 1 oil in the rear differential instead of 140. dont tow much, nothing heavy.
am i protected.? dont feel like draining it. 06 screw 4x4 truck. thanks ! --irv

 

Probably not good when it gets hot...

 

Shouldn't be a problem, at least it's synthetic.

 

I get it. Now you will have to do the job all over again. You are looking for validation. You need someone to tell you, "it's okay, don't worry about it".

I'll echo the obvious. It's the wrong weight. You should use the correct weight fluid. After all, have you ever heard of anyone recommending not to use 75W140, but instead, use 75W90? Well, actually, that has long been debated. There actually are people who think that a lighter oil is better than what the factory recommends.

More importantly, did you use Ford Motorcraft Friction Modifier?

 

Friction modifier is only needed if it's limited slip.

 

Unless you just like walking, it has to come out. The 75w-90 is not a rear gear lube. It's intended to be a manual transmission fluid. It is rated as a GL-4. Your truck REQUIRES a GL-5 fluid.....they are not compatible nor are they interchangeable. The viscosity is semi-irrelevant as gear lube uses a completely different viscosity scale from motor oil. Frankly, at temp the 75w-90 is almost identical to the XXw-140. The problem though is what is in the gear oil and what is missing from the formulation. Sorry, but you need to get this out of your axle housing and fast. I'd suggest cleaning as much of it out as you can. Refill with only the correct GL-5 rear axle gear oil only.

 

I second that.

You could cheat. Remove the fill plug only, and pump or siphon out. Then refill.

Just remember that it is not correct. And you should do it the right way. Remove the cover. Clean it out correctly. Then reseal the cover and refill with correct fluid.

Look on the bright side. This gives you the chance to install a differential cover with a drain plug, dipstick, and fill hole. Most of those covers will allow you more fluid capacity.

 

yes friction modifier was used. sorry i created such a controversy. will pick up mobil 1
75-140 and a tube of modifier. does anyone make a superfast rtv ? since im 70 and
too lazy to look it up how many qts do i need?

funny, this same oil was used before and after i had my 01 F150. now has over 150,000 miles on it.
it also was a 3.55 ls.

 

I suggest you use a gasket. RTV is a pita in my opinion. If oil gets on it before it sets enough, there will be a leak. If you use the proper RTV, it is supposed to set an hour minimum before you put the cover on and it takes 24 hrs to fully cure. Why mess with that crap when a gasket is available for about the same price as the RTV.

 

I beg to differ with you, sir:

https://mobiloil.com/en/gear-lubrica...c-gear-lube-ls

Both the 75W90 and 75W140 are GL-5. I do not think you need to get rid of the 75W90.

 

too late. valvoline syn coming after lunch and motorcraft fr mod. all is rtv'd and waiting final tightening.
thanks for all you advice. truck will only get less than 2000 miles this year, and no winters [4x4].
--irv

 

i used ultra black permatex and waited an hour or so last nov., filled the diff, and no leaks from
then till now. i may wait till tomorrow this time just for sh-ts and giggles.no work in the shop
and its on the hoist. good time for wax

 

Beers To You