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I have a 2000 Yamaha Big Bear 400 4x4. I'm going to change my front and rear differential fluid. The last time I changed it I went with Yamaha diff fluid. I want to switch to synthetic fluid this time. I plow and haul firewood with it. I don't beat it, I do however work it pretty hard sometimes.
I was going to use Amsoil Severe Gear 75W-90. I was going to use this for three reasons, 1- I have more then enough left over from my truck, 2- it works with limited slip diff's (my front), 3- I use their 0W-40 synthetic oil and their Superduty oil filter. Amsoil recommends I use their Universal Synthetic Marine Gear Lube because it works good if water gets inside. I've never had any problems with water getting inside my diff's.
If its "Full" synthetic(NOT RIP OFF BLENDS) its better stuff than dinosaurs remains,grease,motor oil of Gear oil without a doubt. Search goggle "Synthetic VS Conventional" and you will read all day long that Synthetic is Superior to conventional in every way but price.
There are some "blends" out there that cost as much as Full synthetics and are lucky to have 10% sythetics in them,Yamalube and Honda GN4 are a good example.
2001 Yellow 500 H.O
No Plastic roll cage
5.25 gas tank.