Trans can't seem to choose a gear to be in during city driving.

Yea, Mercon 5 is a synthetic blend. If you want a full synthetic tranny fluid I recommend Amsoil ATF... good stuff.
 
If you're going to perform the M5 "mod"(lol) I highly recommend getting a fluid exchange by a machine(see the machine in my thread) http://mark8.org/vbulletin/showthread.php?t=15269.

This will exchange 99% of the fluid vs. the normal "transmission service"(which is only a filter change and 5 qts of ATF) and also if they do a torque converter drain, you're still only going to get maybe 80% Mercon 5.

Do it right or don't do it at all, because you'll just be wasting your money throwing 5-8 quarts of M5 into a 14 quart system. Not only are you diluting the better M5 ATF, but you're mixing it with the leftover burnt fluid.
 
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