Manual transmission oil

I have a 2002 Duramax with a 6-speed manual transmission.
Any input on the transmission oil. Last time I changed it the oil cost over
$200 for 6 qts of the GMC specified synthetic and I had to buy it from the dealer.
Is there a different lubricant available.

The most basic of recommendations is synthetic Dexron III. Ford applications for earlier versions (up to about 2007) of the same tranny call for Mercon V (SP, IIRC, essentially the same synthetic fluid).

Locally, I recommend Amsoil Torque Drive or Castrol Transynd. Transynd is a lot more expensive. Both have performed equally well with no fluid related problems I've heard of. Also acceptable is a quality synthetic Dexron III, such as Amsoil Universal, Mobil 1 Dexron III, Royal Purple synthetic Dexron III, among other high quality synthetics. Stay away from any conventional oil, any "blend", or "gear oil" (GL-5, for example, regardless of base stock). Hard shifting, accelerated synchro wear and overheating are common complaints when the "wrong" fluids are used.

Thanks-
I am pulling the transmission this weekend to replace the clutch. It is about time to change the trans oil again.