According to the rap in the Pegasus Racing cat.  (see
www.PegasusAutoRacing.com  )  the" MTL is a 70w80 gear oil suitable for use
in street and racing applications. >snip< Low temperature flow similar to an
ATF improves cold shifting.
  The MT-90 "has similar characteristics to MTL, but in a 75w90 viscosity. "
Both meet GL-4 specs.
  I ordered the MT-90.  Redline also makes a 80w140, which might work for
really hot climates..or worn transmissions.
  I used a mix of 75w90 and the 75w90NS (non-slip additive) in the transaxle
of my P 928 racer, to try to enhance the limited slip differential's
effectiveness...Didn't seem to stop the wheel spin I could induce with the
throttle at will in the bottom three gears(grin!)
    The Pegasus site:  Has lots of useful parts you won't find with our
Van-specific suppliers or your FLAPs and they are really nice people...No
affiliation other than as a satisfied client.
  Don Hanson