I have contributed a number of times to the discussion threads that involve "what type of lubricant can I use for my whatever?".
One particular thread became rather intense (Tormach 4th axis) even though it did result in some rather informative discussions.exchanges along the way.
Recently, however, we ran into a real PITA situation on our cam grinder. The grinding spindle is sleeve bushed and oil drip fed from a sump tank. Lube was recommended as Velocite spindle oil but we had problems with that and switche do Dexron some time ago.
Things were going along uneventfully until one day the grinding wheel started to "bounce" while dressing. The spindle runs with about 0.0008" vertical clearance and it would iterally "bounce" around (orbit actually) in the clearance space. You could shut the machine down, it would stop, but a short time later, the "bounce" would reappear.
We were using a generic grade of ATF as it was supplied private label to the store by a noted "brand" packager so we didn't think much would/could go wrong. This problem, since it only occured while the machine was warm, led us to do some experimenting.
The ATF we were using was fairly air free even though aeration is an issue - they have a slinger that catches oil and drags it up to a trough and it drip feeds the spindle from the drip fed trough. Again, some "bubbles" but no frothing or what you''d call "aeration". Even so, indications were that temp and cavitation were occurring which would lead to the "bouncing".
We figured that a lube experiment would cost less than $450 worth of new bearings to remove a tenth or so of wear so off we went to the parts store to see what they had in the way of "experimental lubes". We found 4 possible candidates.
Valvoline "high mile" Dexron/Mercon ATF, Mobil 1 Syntetic and Royal purple and a Mobil Blend of base and sythetics.
Ok, time to do some research so, armed with the names, I checked out the internet and read up on the "benefits" of each.
Not wanting to have to disassemble and clean the spindle sump, I elected to try the valvoline "High Miler" stuff. It had 'special additives' for worn boxes and some other additives to help maintain viscosity and pressure under what may be sloppier clearances of used/worn/high mile boxes. It was a bit more costly than the generic stuff but way less costly compared to synthetic or the other options. Again, for less than $10, it was cheaper than new bearings.
We poured it in and fired up the spindle. IMMEDIATELY we noticed a difference. There was NO frothing or bubbling whatsoever in the oil bath sight window. The machine had a different "sound". The proof would be in the grinding. Right from the start, the wheel ground smoother. Interesting....
The proof would come over time when the machine came up to temp. So we let it run to warm up. It did get noticeably warmer (viscuous drag in the fluid perhaps) but no "bouncing". We put the machine back to work and back to grinding some irritatingly difficult profiles - they wanted to chatter, espeically when we do our finish grind where we try for mirror-like finishes.
The difference was noticieabel and just shy of miraculous. The "bounce" was gone and so was the wheel chatter. Valvoline's "High Miler" ATF made all the difference in the WORLD in the grinding smoothness of our spindle. From this point on, the generic ATF we previously used for lubricant we now mix with WD-40 and use as rust inhibitor for finish ground parts.....
In our case, ATF is/was a proven viable oil that we proved/tested as being suitable for our spindle. Dexron is a suitable gearbox lube for many applications as it now is fortified for use in A/T and many manual transmissions and some transaxles with hypoid gearing. It may serve adequately for your gearbox spindle on your mill or lathe (consult owner manual and test at your own risk).
From personal experience, however, there DEFINITELY is a difference between ATF's. In light of this experiment, Valvoline "high miler" ATF is now our lube of choice for our cam grinder spindle as it definitely and noticeably outperformed the "it don't matter generic stuff" we used previously.
My dear departed father once told me, "buy the best you can barely afford, you won't be disappointed if/when you do....".
Good words to live by.... and I should have listened to them sooner.