I use Kluber ISO Flex NBU15 high speed spindle bearing grease. Pretty expensive and you don't get much of it but they do include a nice syringe and assorted needles so that you can inject a small amount of grease between every roller (or ball). I then use a small dab on a clean cloth to wipe over the outer race on a roller bearing. Back when I was doing machine tools this is what we used on our high dollar spindle bearings, good for over 10,000 rpm.
I have seen guys that run the ceramic ball bearings run ATF and have gone over 200.
It all depends on what the bearing load is and the RPM.
Rex