I switched to Mobil 1 ATF at around 34K. I didn't add friction modifier because I couldn't find it. I did find it a year later and had the fluid drained and refilled -- this time with the friction modifier added. This was at 58K.
I found I liked the feel of the shifting better without the modifier, so I changed back to straight Mobil 1 ATF at 67K. I've been driving it like that (no more fluid changes) and now have over 150K. No ill effects due to not using the modifier as far as I can tell.
