Crossfire manual trannys had a factory fill of ATF fluid while the identical tranny in the MB SLK 320 came with 75W80 manual trans fluid.
The DaimlerChrysler owner’s manual allowed 5W20 motor oil as a substitute (although syn wasn’t specified), while MB lists no alternative to their factory fill.
75W80 or 90 trans fluid has a similar viscosity to 5W20 or 30 motor oil - See forum member Kamilspider's Kinematic viscosity chart in the below link.
The Franken-brew mixture you are considering would probably be OK, but there may be some other additive issues/clash with the mix. I think you’d be better off using one fluid. See the below thread with the imbedded links, and see if that helps your decision making.

https://www.crossfireforum.org/forum...-question.html