Synethic or Conventional tranny fluid

I have a 2000 Chevy Malibu LS 3.1L 4 spd auto. Engine has 87K mi and tranny has about 10-15K mi less than engine.Tranny was replaced @ 10-15K mi.
Well anyway, I put on 3K mi in 3 weeks and I average 75 mph but occasionally I feel the need for speed (got it up to 97mph).
But anyway was looking for opinions on synethic or conventional tranny fluids?
Oh, during the summer the tranny slips occasionally when going from park to gear, otherwise tranny is good to go.
Also a mechanic I trust told me to take the car to Chevy for tranny service since the one time they did tranny service on a trany w/o a dipstick (ie, Malibu, ie, 50K mile tranny service) the car wouldn't start and it had to be towed to dealer so they don't service any tranny w/o a dipstick. At least they're honest don't know too many mechanics that'll tell you that. So reading that would you recommend taking the car to Chevy for tranny service?
Appreciate your feedback. Thanks

Hi visit my website and check out my links for amsoil thier you will find info on synthetics. I have a 87 areostar with over 200K ok and i have used amsoil trans fluid from day 1 and i will tell you the aerostar i have has a A4ld trans and they are known to be wore out at 100K or less mine is still going strong so i will tell you that premium syntectics work not cheap synthectics.


I hope this helps you out

rfpoil

http://mylinksite.com/rfpoil