I may have come across a simple solution-- time will tell.
The rubber gasket that holds the viewing window for the transmission oil level has a small groove near the back of the gasket circumference. This evening I installed an "O" ring, sized 5/8 od, 1/2 id, 1/16" wall, in the groove. Before installing, I warmed the assembly up and made sure the "O" ring was well seated in the groove. I then carefully pushed it in place. It took three tries, with the first two attempts forcing the "O" ring out of the groove. On the last attempt, I set one side of the gasket with the "O" ring just inside the channel it pushes into, then eased the rest of the "O" ring in the channel then pushed it on home.
Then I warmed the engine, no leaks. Then went for a short drive-- it's sleeting for god's sake-- and no leaks upon return or 30 minutes later. Time will tell if this is a longer term solution, but so far, so good.
If leaks develop over time, I'll update this post again.