Well you can read all the testimonials, but it boils down to what is recommended in the manual/manufacturer, and what are considered equivalent oils for the specs at the time the manual was written. Their has been a lot written about EP additives, and GL-4 and GL-5 gear oils and issues with brass. But I am not aware of any factual information that states modern GL-4 and GL-5 gear oils will attack or destroy brass gears/bushings. What has been stated in the manufacture literature is that GL-5 has more EP additives, the problem is specific to brass synchros and that the too much EP additives can literally tear the brass off the synchros. Their is nothing about them eating brass bushings/bearings, modern EP additives are buffered and do not attack brass. A synthetic oil will maintain a wider temperature operating range and will last a lot longer then conventional oils. If you look at something like compressor oil, standard mineral oils may last 200 hours, synthetic compressor oils are rated for 2000+ hours and beyond. So if you have a machine that is starting in the cold and then warming up to temperature, then a synthetic oil may be a better choice. In my gear head mill, I picked up about 100 RPM on the top end and it ran a lot quieter when I switched to a similar viscosity synthetic oil. The mill head also ran much cooler.
If you want to erh on the conservative, then go with a GL-4 gear oil such as RedLine MT-90 75W90 GL-4 gear oil or Amsoil Marine 75W90 gear oil. I spoke with their tech people and they specifically said there where no issues with brass gears, bushings etc. I used the Amsoil Marine gear oil because it can takes higher water levels, something that can be a problem over time with condensation in open gearboxes. There are lots of other oils that will work just fine. Most manufactures have specific "Gear Oils" for machinery, not engines, not differentials, not transmissions.... If you do not have a filter and a pressure lube system then the manufactures recommend a non-detergent oil, for the simple reason is that the detergent oil keep particles suspended so they can be taken out by the filter. Use the wrong viscosity oil, then most likely you will have premature bearing/gear failure. There are lots of testimonials on using engine oils in machinery, sometimes they work and sometimes they do not. In machinery that uses splash lubrication, the proper viscosity is important to the lubrication of the bearings, too viscous and the oil won't flow to the bearing. Also viscosity and operating range of oils are application specific, so the ratings and specifications are not necessarily directly interchangeable for and equivalent viscosity oil. This is only at a specifc operating temperature, one that your bandsaw will never get close too.
So the bottom line, I just replace my bandsaw oil with Amsoil Marine 75W90 gear oil because I had left over from my mill, but the RedLine or just about any other "gear oil" of the proper viscosity would have worked fine and would have been 10X better than what came out of the gearbox. On my mill and lathe I use Mobil ISO gear/hydraulic oils that are specific for their purpose. Looking at the quantity, I do not follow the saving a few dollars here or that and experimenting with engine oil. As they say, any oil is better than none, just some are better than others. Do I use GL-1 mineral oil in anything, no.
