some people say it's transmission oil. though i never had the nerve to try that instead.
I have been using trans fluid since '74 when I bought a brand new Sheridan .20. Also use it in my '49 Crosman 100, my Crosman 2200M, my Crosman Mark ll. None of these guns have ever had a seal problem and when I bought the Crosman Mark ll it had been sitting for years. When I put the first CO2 cartridge in it leak right out. So bad I could hear the hissing. On the next cartridge I gave it a shot of trans fluid and that was 2-3 years ago and fixed that problem real fast.My original Sheridan Manual say this about lube: "Any neutral light body oil with a good cold test is satisfactory for use throughout your Sheridan. The average household utility oil sold in all hardware stores and sevice stations for sewing machines, lawn mowers, hinges, etc is perfectly satisfactory."