No, never mix gear oil unless it is an emergency. Use 2.3qt of the Valvoline Syncromesh filled from the correct plug on the distal end of the transmission. It is the plug with the large allen key hole in it. If you fill from the 21mm plug under the shift fork you won't be able to fill the trans all the way up to 2.3qt, which would certainly cause problems.could I use syncromesh fluid along with recline Mt 90
If you're topped up with a single type of gear oil then check your shift bushings in the engine bay. If original, they should be replaced with Brass Shift Bushings to remove any slop from the linkage. You may also consider a short throw shifter for more precise shifts. The C60 is notchy and not the most forgiving transmission, especially when it's cold. So if the car is new to you it may take a few more months of learning to drive it slow and smooth before you get the hang of it. I had a similar experience when I bought a XRS 4 years ago and I've been shifting my own gears for over a decade.