Mobil1 oil leak

[RICHARD KROL]

I change the transmission fluid on my 84 4kq about 1,500 miles ago, now it seems to have a leak.  It appears to have a leak between the center section of the transmission and the rear portion or tail cone. That section is joined with socket head cap screws and they are tight.   I used Mobil1 75-90 and did not over fill.

Has anyone experienced  synthetic gear lube starting a leak?  I have heard where Mobil1 can clean main seals and start leaks.  This is a hard joint where two surfaces a clamped in compression.  

I'm really particular about leaks, how hard is it to change the gasket between those sections on the transmission?

Rick