Rygi

Rear has a slight whine sound after gear oil change. Is friction modifier needed with synthetic gear oil? Friend at shop said it wasn't so I took his word for it. He put in Lucas 75w90. Truck is an '05 F250 diesel with 3.73 LSD. Thanks for any helpful info.

PerryB

That axle calls for 75W-140 synthetic with friction modifier. Put the correct lube in it and it'll probably be quiet again.

whomrig

My owner manual says 75w-140 synthetic with 4oz friction modifier for my F-150
3:55LS

whitescrew77

Your friend is wrong. It needs the modifier because it is limited slip differential. Also need correct viscosity oil put in. Then it will shut up.

Rygi

Thank you guys for the feedback it's appreciated.

Rygi

Well as it turns out he did put the correct 75w140 in, just not the modifier. Apologies and thanks.