So I'm guessing that the oil weight is a personal preference.
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To a degree but the 2W-50 that Polaris now sells weight is good to shoot for. 50 is good for an oil cooled engine like ours but water cooled engines should be fine with more like 40 weigh on the high side. Our 2002 Polaris Magnum Shop Manual calls for the Polaris Demand Drive or Type F ATF in the front hubs is you have a 4x4 on page 7.9. Our front and rear gear cases calls for the Polaris brand or API GL5 80-90 gear oil. Mobil states on their Mobil 1 website for Polaris to use their 75-90 gear oil which I have a bottle of already. In the transmission in seems manuals right before Polaris came out with branded fluids was calling for 5W-30 motor oil.
All this stuff is at WM as far as I know.
If one goes by some info on Bob the Oil Guy site, here and elsewhere you can cause harm. If you do not know machines and lubes it is well to just go with the Polaris brands. No need to put Polaris out of business. We just have equal to or better already setting around.
The best motor oil that I can find for a Polaris four stroke in my view seems to be Mobil 1 0W-50 Racing oil.
As you can read that is due to the 2x+ amount of Phosphorus and Zinc additives. I think this is especially true in the low oil pressure engines like the 400-500 engines where normal oil pressure is 12-20 PSI at 5500 RPM when hot and cams can wear due to lack of oil. Not so much with the 325-330 engines where oil pressure at 6000 RPM is 71-99 PSI when hot but the risk of engine failure is high due to this high pressure blowing not so old oil cooler hoses.
In summary Wal-Mart has the fluids you need but Polaris branded fluids do meet Polaris specs and they will stand behind them better perhaps than some common brand like Mobil 1 from some common place like WM.
Never forget we are ruled more by fear than facts in this life.