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ORIGINAL: barkster99
Am I the only one with concerns about running a heavier tranny fluid? Yeah, I imagine if you get the fluid thick enough, it will help with the clatter. It also won't lubricate like it's supposed to. I'm no fan of HD factory products but I have to think they recommend Formula+ with lots of legitimate research behind it. And heaven forbid you do end up with a genuine breakdown. What do you supposed dealer's first reaction is going to be when he finds out you're not using HD fluid???
Of course, all of these non-HD fluids will tell you that "independent laboratories" have proved their worth. Yeah, right. Show me an '07 with 50,000 miles on non-HD fluid and I'm on board. Until...I'm hesitant. (Though, if dealer can't do something once this snow melts, I may experiment with the rest of you.)
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First, there should be no problem with thicker oil in a gearbox constructed with needle bearings. If it used sleeve bearings there might be a problem with the oil working its way into the bearing and providing proper lubrication.
Next, I used Royal Purple 75w90 in my '96 RK and it went 106k miles without any tranny service, except maybe two pawl adjustments. It never saw HD fluid except out of the factory. It wasn't an '07, but they are fundamentally the same from a lubrication standpoint. The 75w90 gear oil has the equivalent viscosity of a 10w30 (approx.) motor oil, while the 85w140 is the equivalent of about a 20w50. This is a quote from Spectro Oils about using their 85w140 in a Big Twin:
"A Superior GEAR/ TRANSMISSION LUBRICANT designed to reduce clunks while shifting a Harley Davidson motorcycle transmission (no small task) and reduce whine in fourth gear. The 85w140 viscosity (roughly the equivalent to a 20w50 motor oil) dramatically helps shifting when hot. For use in all 'Big Twin' models, any year, without restriction."