I much prefer greased, felt wads between the ball and powder, instead of grease over the ball. The wads are more easily carried and used. The grease is often messy to apply (a Popsicle stick helps).
A mix of beeswax and lard (unsalted, please) makes a good lubricant, or any kind of animal tallow. I can't give proportions because it's been years since I made it. I seem to recall I made it about 4 parts melted beeswax to 1 part lard.
The late gun writer Elmer Keith, who learned to load cap and ball sixguns from Civil War veterans, suggested a 50-50 mix of (unspecified) tallow and beeswax.
For the past 10 years or more I've been using the lubricant recipe named after me: Gatofeo No. 1 Lubricant. You'll find the recipe in my post, "So you want a cap and ball revolver" in the sticky section.
It's a mix of mutton tallow, beeswax and canning paraffin. I haven't found anything better, but you MUST use the specified ingredients. Substituting anything else results in an inferior lubricant.
Anymore, the only time I put grease over a ball is when I'm firing maximum charges and there is no space for a wad, or sometimes when it's so blamed hot and dry here in the Utah desert that the felt wad won't keep fouling soft.
Both of the above are exceptions, not my typical shooting day.