I use 10w 40 like the owners manual suggests. Valvoline is the brand I prefer. Kawasaki wants you to buy another Kawasaki when your Kawi needs to be replaced. They put lots of time, effort, money into design and testing of their products. If your bike doesn't last as long as other bikes you won't buy another Kawi. They didn't think ATF was good in their bikes or they would tell you to use it. I am open to suggestion and not closed minded. I understand new products have come onto the market since the K5 came out, that have surpassed some of the old fluids. Like the synthetic 2 stroke Bel- Ray H1-R that I choose to run in the gas. When the recipe calls for egg you don't add salmon.