I've tried a number of different greases over the years, where grease instead of oil is required, and ended up using CRC Super Longterm Grease (MoS2). It's messy but among the most extreme duty greases you can find on the market and as a big plus, it uses solid MoS2 lubricant mixed in.
It doesn't offer much storage protection but as a slide lube it's fantastic. It has even cured a couple of feeding issues in match grade, tight slide guns. Just dip a q-tip in the tube, apply a light coat, wipe off excess and you're good to go.