Only my opinion, but I would stick to the formula+. First off I really like the Redline products. With that said, when Harley did the redesign starting in 07', the seals aren't quite the same. I've seen a lot of weeping from the seals when people have changed to the thicker oils. Not to say all the newer bikes will have an issue, but you might.
I am a nobody, just my opinion. Like the saying goes "Opinions are like a**holes, everybodys got one".
I have read alot of post here about deffierent seals weeping, and it would be interesting to see how many of those were running "upgraded" oil in the trans/primarys vs factory oil?
I think you will find that most of us aren't running a "heavier" oil, as in a higher viscosity. For instance, Redline Shockproof Heavy is rated at 75w90 on the gear oil scale. 90 on the gear oil scale is roughly equivalent to a 50 weight motor oil. The information I have on Formula+, listed below, tells me that Formula+ is also rated at 90 on the gear oil scale.
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Harley Davidson Formula+
Application: recommended for heavy-duty service in manual transmissions where the manufacturer normally recommends either engine oils or GL-1 lubricants.
Specific Gravity: 0.857
Pounds per Gallon: 7.14
Viscosity, ASTM D 445,
cSt at 40°C: 168.04
cSt at 100°C: 20.10Viscosity Index, ASTM D 2270: 139
Brookfield Viscosity, ASTM D 2983,
cP at -26°C - 28,800
cP at -40°C - n/m
Pour Point, ASTM D 97, °F (°C): -60 (-51)
Flash Point, ASTM D 92, °F (°C): 507 (264)
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If you take the 100°C viscosity of 20.10 centistokes and go across the attached chart, you will see that it correlates with a 50 weight motor oil or a 90 weight gear oil.
Using too heavy a lubricant can be detrimental to the transmission if it doesn't flow to the bearings properly at low temps. I don't think anyone using a
75wXX rated gear oil needs to worry about that. As for the seal leakage, that is much more likely to be caused by the cheap seals H-D uses, and the design issues they created through penny pinching.
Jerry