I need to ad about a quart of auto trans fluid after losing some due to a radiator and trans line change. Car has BB 727.

I'm not sure what the PO used, but almost positive it's not synthetic. I'd bet it's either Dex/Merc, or less likely ATF+4. He doesn't remember.

So, assuming that it's either of the two, what would it be safe to top it off with? I can't seem to get a straight answer from the side of any bottle or from the internet on what the compatibility of either type of fluid is. All conflicting answers...

Really don't want to drop the pan unless I have to for a full fluid change. What are my options for topping off?