Hello everyone. Im switching from 80w-90 to 75w-140 synthetic in the rear diff. My question is how important is it to get out all the 80w-90 before filling back up with 75w-140?

And do i need to fill all the way to the fill plug like with regular 80w-90? I thought i read somewhere your suppose to leave about a half inch before the fill plug. Any help is appreciated. Thank you!

 

Just get most of the old oil out, oil that's left won't hurt anything

No, fill it all the way up to the hole

Most synthetic has a friction modifier already added for Ford or GMs Limited Slip differentials, i.e. trac-lok or positracion

If rear axle starts to chatter a bit then add some

 

Thanks for the response! The job went smoothly! Lets hope for no leaks...