I caught this today at a web site that also sells airguns:
“While RWS spring-piston models do rely on a tiny diesel effect to produce full power, sever (sic) dieseling must be avoided.”
Elsewhere in this company’s web site they recommend that “One or two drops every 5,000 to 6,000 rounds, or each 8-12 months, should be plenty.”
Modern airguns are lubricated with very tiny amounts silicone-based oils that have a flashpoint of between 450 and 550F, and simply do not burn any significant amounts of oil- unless the owner squirts excess down the barrel, as Beeman still recommends. Many guns use a silicone-molybdenum disulfide paste that will not vaporize under any circumstances.
Supposing the oil did burn. Given that they say a properly lubricated spring piston airgun can fire at least 5,000 shots before needing a tear down and relubrication, how much oil do you imagine could burn on each shot? Those two drops of oil- about 100 mm^3, weigh roughly 7.15×10^-5 grams, which, divided by 5000 shots comes to 1.43×10^-8, or 0.0000000143 grams per shot.
You could be lubricating the gun with gasoline, and you’re still not going to get very much energy from 0.0000000143 grams per shot! Note that Beeman, who used to recommend two drops of oil with every tin of pellets, now say that:
The piston seal in most modern air guns is made of a synthetic material that is self lubricating. It should only be lubricated during routine maintenance performed by an authorized service shop.
So there you have it. No oil, and no dieseling.