Trying to find propper oil for my mill.

Those are all engine oils. Wouldn't a 50w transmission oil be best?

Taken from Mobil Delvac Synthetic Transmission Fluid 50's description sheet, ( https://www.mobil.com/English-US/Commercial-Vehicle-Lube/pds/NAXXMobil-Delvac-Synthetic-Transmission-Fluid-50 ) "It has the same viscosity as most SAE 50 engine oils and SAE 90 gear lubricants at the high end of the temperature-viscosity scale, and because of its inherently high viscosity index, it provides stronger film strength at higher temperatures than conventional oils. Mobil Delvac Synthetic Transmission Fluid 50 also ensures effective lubrication at below freezing temperatures without channelling."

or, my favorite amsoil50.jpg
 
I would be very careful about mixing synthetic oil with conventional oils. It's fine in car engines because the SAE standard requires
oils to be compatible with one another. With other oils that may not be the case. Worst case scenario is that mixing them results
in bearing failure. My son works in a shop that had to rebuild a couple of escalator gearboxes for that very reason.
 
I wasn't suggesting mixing syn/non-syn. Just wondering if he shouldn't be looking for gear lubricant rather than engine oil.
 
Usually older machine tools use GL-1 machine oil. GL-1 oils are straight mineral oil, that contain no modern additives. The additives in modern formulations all contain rust and corrosion inhibiting additives that are harmful to yellow metals, such as copper and brass that were used in seals and bearings of older machine tools, such as the VN 12. Each grade of hydrolic oil from GL-2 thru GL-5 contains increasing quantities of additives, and should be avoided.

The GL rating is printed on all OTC lubricants sold in auto supply stores and elsewhere. so not hard to find. On the west coast, I’ve always found Napa and several other suppliers commonly stocks a supply of GL 1 oil. These continue to be used in old car transmissions, as well as machine tools.

Glenn
 
SAE50 motor oil is equivalent to an ISO220 gear oil. The recommendation for SAE50 was probably in a time before ISO was used in the US.

Traditionally SAE motor oils are designed to run with pressurized lubrication and a filter, so usually have detergents to keep the particles in suspension and also the viscosity index changes dramatically based on operating temperature. You would probably be best off with a gear oil. It is true that the higher the GL rating the more EP additives, historically these were known to attack yellow metals because of the use of activated sulfer. But current EP additives use inactive sulfur which is activated with temperature, still when softer or dissimilar materials mate with EP additives they can cause damage to the softer metal. Other issues with automotive gear oils for differentials and transmissions is they are often designed for both diffierent operating temperatures and also for limited slip/synchronizers, so may not be optimum for a machine gear drive.

There are benefits of the other additives such as rust inhibitors, anti-foaming, etc. On my last gearhead mill it specified a similar ISO/Gear head oil 80W-90 and after it was broken in I completely drained the head and ran Amsoil Synthetic Marine Gear Lube 75W-90. I picked up ~100 RPM on the top end and the mill head ran much cooler. The oil is designed for marine gear drives and does not attach yellow metals, the added benefit of decreased foaming and not being affected by water contamination. I would recommend using a good quality ISO220 gear oil specifically for machine drives such as Mobil SHC-630.
https://www.zoro.com/mobil-mobil-shc-630-circulating-iso-220-1qt-120272/i/G0943074/
 
One other thing to avoid are hypoid gear oils, such as are used in modern automotive transmissions. Hypoid gear oil are the gl-5 oils that contain the extreme pressure (EP) additives. Although these do inhibit foaming and rust they have the property of having a very small molecular structure. This results in the oil being to ‘slippery’ for old style gears. Essentially the oil runs off the gear surfaces and does not lubricate properly when your machine is in operation. Basically they do not provide a proper film of lubrication except at extremely high pressure - which our older machine tools do not produce. The result is abnormal wear in your gear box.
 
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