I *like* the idea of drain plugs in transmission pans - A worthwhile mod, IMO
Years ago we had a Mitsu with an automatic which was known to fail, often before 100K was reached. It came from the factory with a trans drain plug (which I'm sure 90% of owners never made use of) and every other oil change I would drain and refill the trans - I was changing engine oil every 3K with conventional oil. The trans took just shy of 3 quarts to refill after a drain. This meant (to me at least) that I was doing a complete fluid change every 20K or so, which I'm sure had to be a good idea for a transmission known to have 'problems'. Our car was totaled by a drunk driver when we had about 90K on it and we never had any trans related problems, so money well spent, IMO
If you have to drop the pan to install the drain plug, why not change the filter too? In my not so knowledgeable opinion, if you want to switch from conventional ATF to synthetic, I think you should do as close to a 100% change as possible when you make the switch - Not by adding 3 or 4 quarts once every year. I'm sure they'll say the conventional and the synthetic are 'compatible' but running with 3 or 4 quarts of new synthetic mixed with 5 or 6 quarts of old 50,000 mile conventional doesn't sound like a good idea to me. I just bought 16 quarts of full synthetic to make the switch on my TC, but I recently realized that I'll need about 4 quarts more to get a 95% or better switch over
If you only want to change 3 or 4 quarts regularly with your new drain plug, I think I'd stay with the factory recommended conventional ATF - Just my opinion
Don