I'M putting mobil 1 75-140 100% synthlic do you want to submerge the gears? last time I put to many Qt. bottles in when I came back from vac.a oil pool in the back of the truck!.
If you fill it from the plug, fill it til it hits the top of the plug and you will be set. You may have a bad gasket/seal on the rear cover if it is leaking