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Q: Rear differential

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I had my rear differential service on 7/4/16 and the shop put 80w90 in it. &not the 75w140 synthetic that ford calls for. Lest then a day my truck rear diff started leaking even more then before & now the case got hot. I stop driving and waited until the next month 8/4/16 &had another shop well call it shop 2 . Well shop 2 found mental shaving, hardly Any oil & a bad pinion seal. I called shop 1 they said they wouldn't do anything for it. So shop 2 had it all that fix . The truck ran good until about 1 1/2 weeks ago where the sound started coming back . I baby the truck and last two day the noise. Got louder so I stop driving took it back to shop two. They found alot more shaving it it again. But there was still lots of oil. This happend today 2/8/17. And know I have to replace my rear differential all together. I know they put the right oil in it.& the limit slip addtive. My question is do you think the 80w90 messed up my truck even through I got the oil replace in a month time?

My car has 105000 miles.
My car has an automatic transmission.

The service manual does not call for the differential to be serviced unless there is a leak. You do mention there was a pinion seal leak before first service. The pinion seal will leak when the pinion bearing is bad. The bearing will continue to leak even if the seal is replaced and will continue to shed metal into the housing. The thinner oil would not have caused the problem if it was leaking before the first service.

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