Has anyone used some version of "No Slip" in their tranny? I'm getting some slippage b/t 3rd and 4th gears (automatic). Will doing this perhaps raise other issues/problems??

 

How do you know you're getting slippage?
And if you are it usually means a clutch pack is going to go soon or a solenoid may not be working....all of which a can of whatever will not fix in a million years.

i would check the fluid, maybe even flush it and replace with fresh fluid.