[brucepick]

I'm happy replacing one tranny pan's worth at every 4K mile oil change. Synthetic, that is. Would work fine for non-synthetic also, I'd think.

I've been changing out one tranny pan's worth of my ATF at every oil change. In my case that's 2.5 qt of the 7.8 qt tranny capacity. Pull the plug, let it drain, replace, check to verify the level next time driving.

I'd already changed over to synthetic ATF before I started the regular changes along with oil change.

My theory is - the owners manual says to change out the ATF at 30K miles. So I change 32% of it approx every 4K miles. After 8 changes (32K miles) I've changed 95% of the fluid. I'm happy with that. The synthetic stuff is pretty pricey, about $6/qt last time I got some. This spreads out the cost so it's tolerable, and gives a mix of old/new fluid.

Before you start thinking how bad it is to be running around with half your fluid older than 15K miles, just remember that waiting to do a full flush leaves you driving with 100% of your fluid with um, how many miles on that stuff?

If I keep the car long enough I might do a full flush after maybe 100-120K miles. But I think it would be fine if I never do that full flush.