On our transmissions, a Redline MT-90 and Syncromesh blend is very commonly done due to the issues that newer GL-5 gear oils present for our gears. TL;DR, the new stuff has high amounts of sulfur, which eats our brass synchros. This made it so people had to find an equivalent GL-4 oil that was free of sulfurs. MT-90 worked best, but was well known for being pretty notchy while cold, so often people would blend in a bit of Synchromesh to allow better cold shifts. This is the mix I use as well, and it's done nothing but good for me. I run a 1:3 ratio of Synchro to MT, and at the very least it made the grind a bit smoother and allows me to granny shift without grinding.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.
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.
FWIW @Meatbag44, I also have a third gear grind. If you don't want to rebuild it, and if the grind isn't that bad, you can just make sure the gear oil is clean, and then baby that specific gear until a rebuild is easier for you (hint, shift a bit slower).