Harley only puts that in there because they wan't you to buy THEIR overpriced oils. Just like when they used to say not to use synthetic oil in your bike. That was until they started selling THEIR brand of synthetic oil. It has been proven for years by people, using just about every brand and type of oil made that Harley's aren't that delicate, or picky about what oil is used in them. Some work better then others, and most work better then what Harley wants to sell you. I've been using the cheapest Gl-5 rated gear oil I can find in my bikes for years with no issues. And I've been running Rotella t diesel oil in my primary also. I have one of the quietest, smoothest shifting Harleys out of anyone I know. Remember that every bike manufacturer buys it's "branded oil" from the lowest bidder. So how can that best the best there is?.,,,