I'm still trying to figure out why I need to use the heavy.
jimp,
I searched through my old stored emails, but couldn't find the one I received from Redline many years back. They stated that their recommendation for H-D transmissions was the Shockproof Heavy. The following descriptions are from their web site:
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Heavy ShockProof™
A unique lubricant containing a suspension of solid microscopic particles as an extreme pressure agent--unique solid dispersion which cushions gear teeth to help prevent tooth breakage and allows the use of lower viscosities. Recommended for heavily-loaded racing differentials and transmissions, Off Road racing and problem gearboxes. The viscosity characteristics allow the lubricant to resist throwoff and provide a film thickness similar to a 75W250 grade, while providing the same low fluid friction as an SAE 75W90.
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LightWeight ShockProof™
A unique gear oil designed to lubricate racing transmissions and transaxles which see serious loads (not recommended for most syncro-type transmissions). It has excellent low-temperature flow which allows easier shifting when cold. May be used to obtain maximum power transfer in racing differentials which do not see high temperatures. Similar to a 75W140 gear oil, but with the lower internal friction of an SAE 30 motor oil.
Used in two-stroke and four-stroke motocross transmissions (often mixed in equal parts with SuperLight ShockProof™ Gear Oil).
Used in most road-racing differentials with moderate power and dogring racing transaxles.
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Jerry