I come from the high vacuum field. Yes all HVP oils are straight mineral oils, other than true exotics like Fomblin. And yes they are carefully refined so that only long-chain molecules are left, resulting in vapor pressures in the .1 micron range or so. There are two general viscosities: Belt-drive, of higher viscosity; and direct-drive, of lower viscosity because the pumps turn faster and tend to overheat on belt-drive oil. Most of it is yellow-ish. I'm embarrassed not to remember if these general-purpose oils have any discernable odor in the headspace of the container: I think they do have a slight odor. Occasionally I'll splurge for "Technical White Oil", which brand I buy, at least, is water-clear. These have been further refined, or synthesized to bar all aromatics, and are odorless.
The ideal GP vacuum pump oil is hydrophobic. Water in the oil is a contaminant, no more welcome than the water which eventually limits the performance of deep frying oils. Ideally, it drops to the bottom of the pump housing, where it often wreaks some havoc with iron casings and sometimes drain plugs if not attended to. Normally this would need draining on occasion, and because it sinks can be drained off without necessarily replacing the oil charge. But it nonetheless does contaminate the oil, and would limit the ultimate vacuum that can be achieved... were it not for the GP vacuum pump's secret weapon: the ballast valve. The ballast valve admits some air to the second stage. The second stage is chosen in part because the first stage buffers its effect from the system under vacuum, essentially turning the pump (partly, as the second stage is not fully saturated with air) into a single-stage pump while the ballast is open. The degraded ultimate vacuum rating with the ballast open is also generally specified. Technical pumps are often rated for water vapor throughput: in grams/hour or sometimes as a function of the partial pressure of water at the inlet of the pump: sometimes both. This is the continuous rating with the ballast open: within those guidelines, water will not collect in the casing of the pump.
These pumps are rated for and can actually achieve base pressures in the .1 to .2 micron range.
The air admitted to the second stage is compressed and then re-expands once it goes through the exhaust valve. This "work" tends to heat the air directly, and indirectly the pump oil, which aids in the carrying of water out of the pump oil. The air is dispersed very finely into the oil, making for a large surface area and making it milky. But these little bubbles tend to rise through the oil bath in the housing, whereas the pump draws oil from near the bottom of the sump: the moist air tends not to recirculate per se.
I use a purpose-designed catalytic trap on the inlet of my under-the-bench pump to guard against pump oil backstreaming into sensitive apparatus. At the ultimate pressure of the pump, this oil competes effectively with water vapor and air (and the water vapor in the air) as a limitation to the ultimate vacuum of the pump. Some improvement in ultimate pressure can usually be had by purging with dry air, but the backstreaming problem remains. As I recall, this trap generates a gram of water or so per hour. So when I notice base pressure deteriorating, about once an hour or so, I open the ballast for a couple of minutes. I run my exhaust through a clear PVC hose: it mists up with vapor and droplets at first: when the hoses run clear again, I know it's finished.
Now enter the HVAC pump. These very rarely have ballast valves.
But keep in mind the usual wisdom in the HVAC world: 250 microns is often used as a rule of thumb for "good enough". This places lower expectations on the pump oils. Having no ballast, oil changes are expected to be at least somewhat more frequent: it's appealing to have less expensive oils.
Occasionally marketing plays tricky roles. If Yellow Jacket advertised that their oils were good to .1 micron, failure to achieve that base pressure with their pumps would tend to impune the pump, not the oil.
Some pump oils, esp. into the HVAC market, do advertise that they are hygroscopic: however, I know of no additive that is added to any pump oils to make it hygroscopic: they all champion their purity.
I highly suspect that these oils are no more hygroscopic than any GP vacuum pump oil. But because HVAC pumps have no ballast, they promote this inevitable but undesirable property as a desirable one, if only to better rationalize the absence of ballasts on HVAC pumps.
Back to the OP's question, it is not likely that compressor oil would be suitably refined for use in technical applications. But if it is virtually odorless, and has no adverse additives, it would more than likely be suitable for HVAC applications: probably down to at least 25 microns, I would guess: setting aside the issue of viscosity and possibly heating the pump (and increasing backstreaming) to some extent. According to one theory, this unrefined oil could continue to contaminate the pump across a change of oil or two of high-grade oils. But it's not quite that bad. To some extent, the base pressure settles to the average vapor pressure of its components. Made for-purpose pump flushing fluid, usually red in color, presumably has a higher vapor pressure if not some downright solvent in it. Draining the fluid and replacing with GP hivac oil is considered sufficient to restore pump performance.
