I recently came across a place to buy top quality (Specially made for Airsoft) cylinder and molybdenum gear grease. It was purchased from Tirador on ASC (the same guy who designed the Sorbo Pad for AEG gearboxes). I compared both Systema and Tiradors brand of cylinder grease and they both have about the same consistency (Systema is a little thicker) and both provided the same air-seal.
Systema cylinder grease is expensive, about 20 bucks for a small container. With this brand you get about 2x as much for about half the price.
Price is $10ea. (250grams per plastic container) plus shipping.
Molybdenum gear grease is 10 ea. plus shipping. Pics to come.