Hey Scotty, I buy only used cars, most have no service records. Some with around 100k miles. I like to keep the trans fluid fresh by draining, "with a pump" a quart or so, and refilling with fresh fluid, and some Lucas transmission additive every time I do an oil change, until I go through a gallon or two. Thoughts?
I wouldn't put any Lucas additives in your transmission unless it's slipping. You said you change the transmission fluid in your cars at ~100k miles. What cars have you bought used and what do you currently drive right now?
No need for Lucas except as a last resort band-aid for a slipping transmission.
What I do is a drain and fill of the pan every other oil change (2x5000 miles), and do a triple drain and fill every 30,000 miles along with changing the filter. A little aggressive but the transmissions in my vehicles use inexpensive Dexron II or III fluid, and the fluid is a lot cheaper than transmission repair. (Also my daily driver has an automatic trans that has almost no support in the U.S. any more, ZF 4HP18, so I really don't want any problems.)
what about the filter?
