Decided to install MM6 again after a while, and noticed my fans getting really loud when in the 3D world.
Went to check temperatures and saw that the game is causing my CPU to shoot up to over 80c degrees.
Note: It doesn't happen with MM7 or MM8.

Has anybody else experienced this, or knows of any causes/fixes?
I don't remember this happening in the past.

Do you have VSync enabled (you might want to check video card options)? If not, then enable it and see if problem goes away.

Oh yeah I've tried limiting framerate, turning on vsync and tweaking compatibility settings in properties.
Perhaps this is just normal and I never noticed it before?

Do you have any unofficial patches applied?

If not, how does it run if you make fresh install and apply GrayFace patch?

I must say that MM7 and MM8 run somewhat slower with GrayFace patch (so this or similar unofficial patch also might be the reason of higher temps). But on other hand, MM6 (unlike MM7 and MM8) acts very weird on modern PCs without GrayFace patch, even causing blue screens of death, etc (GrayFace patch fixes that). Perhaps you don't have BSoDs but have increased temperatures instead and that might be fixed by patch.

You can get GrayFace patch from here:
https://grayface.github.io/mm/#GrayFace-MM6-Patch