I use 75W90 synthetic hypoid gear oil (actually quite unlikely you would find an 80W90 or 75W90 that is not a hypoid oil, if it is for the rear axle of any pickup truck, it is a hypoid oil). You may also find after changing your front axle gear oil that you have a creaking noise when you take corners if you have the Honda limted slip front axle.. You can use automotive oil in your atv .I got a honda atv that is 28 years old .That My dad bought new and i have ran Penzoil 20w50 in it .I have rebuild it one time and never had a clutch in it .I run 15w40 Rotella in all Rhinos.I run 10w40 penzoil in my new 420 rancher.But i change it every season.If you dont wont to change it very much run Mobile 1 Synthetics and change it every year.
The front diff is different. The oil fill cap is down lower on the side of the diff. On the front diff, just pour the gear oil in the oil fill hole and when it comes up to the bottom of the hole, it's full. Of course, the drain plug is on the bottom of the front diff, as well. Both diffs use hypoid SAE#80 gear oil, any brand will be fine.. How to Change Honda Rancher 420 Diff Fluid. Changing the final drive and front diff oil in a Honda ATV is an often overlooked maintenance task. A Honda Rancher 420 takes the same type of fluid on both ends, and you should change those fluids once every two years, regardless of mileage.