Transmission manufacturers work very closely with automobile manufacturers to design a transmission around a specific fluid to ensure maximum lifecycle and operating performance. This is especially true for Getrag. When the OE mfr sells an application-specific special synthetic manual transmission fluid for $20 per quart (XT-M5-QS), it's obvious to me that you aren't going to find a more suitable substitute. A slight change in lubricant characteristic can result in premature transmission failure.
Experimenting with non-approved alternative fluids is something that made some sense 20 years ago when the OE's were running $4 per qt non-synthetics. In those cases a synthetic could sometimes be substituted with better results.
Redline destroyed many, many Getrags and Borg Warner Transmissions in the last 2 decades. The minimum wage morons on their phone tech lines used to recommend MTL for everything under the sun without any scientific basis (testing or verification). Royal Purple marketeers follow a similar strategy putting the consumer at risk. Fact is, Redline does not abide by the most basic API certification processes so their produsts are approved for almost nothing. These unapproved lubes are designed for racers who are willing to accept accelerated wear and yearly rebuilds for tiny fractional horsepower gains. Toyota Supra owners and Ford Mustang owners lost trannys at an alarming rate due to serious incompatibility with MTL. The OE mfrs got involved because these issues were causing failures during warranty period. Toyota trannys jambed up from parts swelling and synchros delaminated on all world class Borg's.
Experimenting with non-approved fluids is very bad idea. Comparing a couple oil specs means nothing.
The recommendation sleepyboy made to use 75W-90 is ill-advised. ALL 75W-90 hypoid GL5 lubes contain caustic agents that will destroy modern "yellow-metal" and/or composite synchros. The posi clutch anti-chatter additive has nothing to do with it. That comment makes no sense. Nobody should be running 75W-90 in a tranny designed within the last 30 years (80's and up). Please don't sticky this thread because it contains terrible advice that serves to destroy hardware.