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I think Coltri would be the people to ask but in the USA the owner of this website (I have no connection with them!) has most of the answers.
Go to:
Coltri Sub High Pressure Breathing Air Compressors Supplies from Ray Contreras Sales
Its 54$ to the gallon. Expensive but not ridiculous.
Seajay
"751" is a designation for a diester synthetic oil produced by Ultrachem, Chemlube 751. A replacement would be Chemlube 800, which I believe is a triester synthetic.The simplist way, is to ask the supplier for the MSDS (Material Safety Data Sheet), for the oil product you are asking about. You can compare these with MSDS from different manufacturers/suppliers. If an oil has the same characteristics, it is a good replacement. Most oils offered by compressor suppliers are relabeled anyway. Suppliers buy drums of oil from a manufacturer/distributor and package it under their own "brand".
Craig
This was on the MSDS supplied after contacting the relevant companies and reading the info provided, usually very extensive. They all showed ISO, SAE, flashpoints, spec gravity, density and more...The one thing interesting note was that some of the equivalent oils were noted as "not for breathing air" applications. Some also stated they didnt go well with too much O2.
R