OEM grease for Omega air compressors (made by GX I think) https://renewablelube.com/products/bio-food-grade-e-p-grease Not cheap. Can also be found on ebay.
You might want to check the entire regulator housing with some soapy solution before you tear apart the receiver. I have had two (yes two) that developed a microscopic crack or fissure in the housing block itself. AOA replaced the first one and I had to replace the second one on my own dime...
Your slugs may be seriously undersized. The Huben mag is notorious for having oversized mag chambers and slugs can fall forward too far forward when it cycles the mag.
Before I began seating my pellets fully into the mag, my mag would jam while rotating it if the gun was pointed upward. There is a ridge in each chamber of the mag that the pellet must be seated into. You will feel it seat.