Title pretty much says it all, but here's the background:
I had my 1997 Blazer (4.3) in the shop to fix an oil leak. They identified the leak as a small hole in the oil cooler lines and replaced them. Two techs were involved. The first tech thought it was a transmission leak, his evidence being that I was low on fluid ...one problem, the leak was brown fluid, not red, so this made me doubt his talent. Fortunately, the second tech stepped in and found the actual leak at the oil cooler lines. So I decided not to worry and authorized the repair.
In addition to the oil cooler line replacement, they also topped-off the trans fluid. As per usual, I drove a short distance down the street and checked both fluids while doing a quick inspection for post-repair leaks, for example, from a drain plug. I noticed BOTH the oil and the trans fluid seemed a little overfull, but I thought, "well, they're the professionals, I'm sure they know what their doing," and I went on my way.
[Aside: I actually had to return the vehicle the next Monday because it started leaking again. Profusely. It turned out they reused the o-rings at the oil cooler line fittings, and these didn't seat properly. Fortunately, they were honest with me and properly fixed it at no added cost.]
Fast forward to now, about a week later. I have probably driven 1500 miles since that repair, mostly highway miles, and I'm seeing a transmission leak. It's definitely red trans fluid. So I called the shop and spoke to the better tech, and he ensures me that these are "tough transmissions" (mine has 245000 miles, original power train) and it's most likely coming from an overflow vent, and that I can bring it by and they can extract the overage.
What worries me, though, is I've been researching online trying to identify common reasons for trans leaks, and guys on the forums are insistent that overfilling a transmission can be a sort of death-blow...and this has me particularly concerned that my old, aged system would be even less likely to resist such damage.
I apologize for the winding back story...but I thought the character of the technicians might have some bearing on your answers. Is the guy being straight with me? Is my Blazer transmission relatively tough, and the new leak I'm seeing is just seepage from an overflow vent...in spite of having put 1500 miles on it while overfilled?
Update, Nov 2018: I don't want to jinx it, but it's almost exactly two years later and this transmission is still holding together. Current mileage is 273k. If memory serves, I had the shifter seal replaced, and I have not seen any notable leaking from the transmission since this incident. ::knocks on wood:: ::crosses fingers::
Update, Jan 2023: A new answer on this post led me back here. This vehicle is still running! ::does superstitious good luck things:: Currently at about 284k miles. In 2019 I picked up a 2002 that was in good shape, and between it and the pandemic, a lot of pressure was lifted from the '97. At this point, oil leaks in the '97 are the bigger issue. I think one of them is the rear main seal. But the trans seems to be hanging in there.