Kaeser screw compressor oil

I've ended up with a 2001 Kaeser SK19 screw compressor that was one of a pair from a small manufacturing facility, unknown history, sitting for about the last 6 years. They robbed a sensor off of this one and drained the oil off to remove it before I ended up with it, now I need to replace the oil. It holds 3.25 gallons of Kaeser sigma m-460 oil according to the sticker. Presumably that is what is in there. I'm told the unit holds a good bit of oil internally that remains inside even after draining so compatibility with existing oil is important even if I drain what may remain. I don't even know if this thing works, so I'm not too anxious to buy a $250 5 gallon bucket of oil just to test it, so I decided to do a bit of research on screw compressor oils. From a bit of googling I think the Kaeser m460 is iso 46, I was told alternate compatible oils are Chevron gst46 (gas turbine mineral) , Castrol Hyspin aws46(mineral oil), Mobil dte medium, Exxon Nuto h 46 (looks like mineral), and Shell Rotella 10w/Shell comptella oil 46/ Shell oil S.9159. I'm told kaeser oil is not compatible with food grade or sullair oils. Looks like a bunch of compressor dealers have found whatever the secret stuff is and repackage some generic oil and mark it up to sell as compatible by brand. Is this stuff just iso 46 hydraulic oil? Additional googling indicates ATF is among the best quality of hydraulic fluids, but never seems to list a viscosity in specifications. So whats the difference between Kaeser m460, iso 46 hydraulic oil, and ATF? They all are around the same price...I suspect the rotella 10w may not be the common rotella straight grade 10w engine oil or am I wrong? that stuff or regular iso 46 hydraulic oil would be ideal to test this unit with as I can probably pick it up locally cheap, and if the machine works I can drain, do a filter change, and refill with the OEM kaeser stuff. Opinions?

You've got yourself a potentially great compressor and you want to cheap out on the oil??? Taken care of, those things run forever! We've got a 50hp Kaeser with 30,000 hours on the clock still running great. I use only OEM oils and parts.

you missed the point. The first oil change will essentially be a flush to be replaced with the OEM stuff if the machine works...

I maintained big screw compressors for years........if the machine will see heavy use,then the result of using unsuitable oil will be coolers blocked with shellac......I dont know if its actually shellac,but it is a flaky solid oxidation product of oil breakdown......I suggest you remove a small filter,and cut it apart,and look for these flakes.....if there are none,then all is good......if filter is blocked with flakes ,then the coolers will need cleaning.....We used a ISO 68 grade R&O turbine oil.....but its hot here,and the compressor building was always 50-60deg C inside,and the blast from the cooler just like a California bushfire.

The oil separator is in effect a large filter,and if it gradually fails thru mechanical wear ,and blocking........with a screw compressor,any dust that goes in thru the air cleaner filters is circulated in the oil until trapped by the separator.......when the separator fails,it develops a "bare spot" or tear that allows liquid oil into the air system,causing problems ,to say the least........the other potential issue with screw and vane compressors is massive internal fire if a bearing fails and runs red hot.Dont run with noisy bearings.

Our Kaeser manuals list two types of oil,the cheaper one is a semi synthetic.Not at the shop now so can't give you the pn.It may not be that much cheaper anyway,don't know,never used it.
I don't see how you could damage the unit using standard aw46 oil for a short time just to check it out,just be sure you can drain it ALL out and clean the separator.ATF has friction modifiers in it that compressor oils don't.It will stand the temps ok.
If it were mine I'd test with the DTE and if it worked ok then clean and change to the good stuff.

All we ever used in 2000kw+ of compressors and diesel portables was a R&O 68 Steam turbine oil.I know there are a lot more costly oils said to extend change intervals(1000hr with me),and I think Sullair even market a synthetic that doesnt need changing.However ,oil is lost in screw comps to the system,and also in maintenance operations like separator changes.I cant see the value of synthetic unless it works in very hot conditions......diesel portables run much hotter than electrics,yet the same oil was used in them as the electrics.....i would say check the filters,and if scale is forming,then go to a more oxidation resistant oil.