What is the best make of grease for driver cellars on a mainline locomotive? Source? TIA

Texaco Hy-Tex 8. Unfortunately no longer available. Some of the current operators of grease cellar engines can chime in with what works for them. Are you doing mainline running at track speed or ?

Steam Services of America sells 99% of the grease for the steam operations I have been involved with. I'm sure you are aware an oil cellar conversion for the drivers is also something you can look into.

If you describe a little more what are looking to accomplish, you will likely get many good responses and suggestions on here.