Soluble v straight cutting oil in low humidity

Hi All,
I live in the AZ High Desert and it is very low humidity (a bit high at 31% at the moment) all year round. I have to replenish the water/
oil mix regularly as the water evaporates out pretty quickly. I was about to order some more soluble cutting oil and thought I would ask here first.
I am wondering if a straight cutting oil like Mobil 404 would last longer, maybe indefinitely. I mostly cut aluminum on mills and lathes so do not want any staining after machining.
Anyone have some low-cost favorite cutting oils?
Thanks
Dave
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Im semi-seriously considering the same thing myself--I'll be cutting aluminum only except for a very small amount of brass--I've found online refrences claiming that using "universal tractor hydraulic fluid" is about as dirt
http://www.tractorsupply.com/accel-303-tractor-fluid-5-gal--8060004
cheap as it gets and is actually fairly effective....which seems reasonable so long as you back off a couple notches on the SFM and make sure to maintain adequate chipload on the so as to not generate / concentrate excessive heat and thus cause a meltdown...
I suppose I could also blend in some percentage of a commercially prepared cutting oil if need be...
Anyone else have thoughts or opinions on this ?
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Hi,

That's an interesting comment. I was reading about Mobilcut 102 and Mobil say it can be used as a light to medium hydraulic fluid. I assume not mixed with water that is.
http://www.mobil.com/USA-English/Lubes/PDS/GLXXENINDMOMobilcut_102.asp
I will check out the hydraulic oil. You might want to check out the data sheets before you decide as Mobilmet 766 cutting oil is corrosive to copper and it's alloys.
Thanks.
Dave
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about
Enco has a special running till the end of month on their house brand cutting oil $52.95 per 5gal tub and it appears to include free shipping.
http://www.use-enco.com/CGI/INSRIT?PARTPG=INLMKD&PMPXNO0603&PMAKA5-857 0
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