Th 400 tranny question

[Sierra sin fin]

My 85 GMC has been having some transmission problems. My TH 400 transmission has 140k miles on it, the engine 60k. This summer it started occaisionally taking a few tries to get into park- I would put it in park and the truck would move with a clicking sound. A try or two or three would soon result in a slight clunk as park engaged, then that three ton plus some truck would stay nicely in place. I took a look at the linkage, and as nothing seemed ready to fall off, I chalked it up to a worn linkage. Not the most thorough troubleshooting. It has been leaking fluid of late, and I see a drop of anti freeze on the bottom of the flywheel cover (anti freeze goes down slowly). Recently, on a hill, it just was out of gear when I went to accelerate. Not a trans mount or engine mount problem, I think, as there was nothing as I touched the gas. It had magically gone from drive to neutral, it seemed. Some fooling with the shift lever got me going after a few stressed moments. That has gotten my attention. The fluid was really low, which was funny as I had added a pint a few days before. A quart seems to have stopped that problem. My next planned step is to adjust the linkage and maybe replace the half moon bushing (not sure where it goes yet-just found out about it on the phone from the local Chevy/GMC parts dept ) and I am considering adding a bottle of the Lucas transmission renewal stuff they sell at NAPA.

My questions are these: Has anyone had this sort of problem? What was it? What might it be? And has anyone tried the Lucas additive? Did it help? Do I seem on the right track? What else might I do?

Thanks for reading,
Dave