Hi guys, I came across this YouTube video the other day. I thought you may find it interesting to see how the heat build up varies between a lighter and heavier viscosity transmission oil. This is one reason why I've always believed that a thicker oil is not always better. See link below. 

That's a neat video Baron, I'm not gunna go into it too much because oil threads turn into a joke after awhile but I will say this, the vid is not the whole story, there is a lot more going on that needs to be accounted for, unless you really know, and I mean really, stick to the recommended viscosity's for your climate. 

Yeah Hilly, I had no intention to turn this into an oil thread battle. The video was probably done in a controlled lab environment but I could see that in actual hot weather riding the temperature differences between the two weights could be even wider. Both oil viscosities will still protect the same, just the thicker oil will run hotter because of extra drag. 

What is the recommended operating temperature range your transmission should be in to meet the manufacturer's design parameters? 

Very good question, I don'nt think I've ever seen any information on transmission or differential oil temperature requirements. If you look at a heavy-duty truck gearbox and differential, they mostly use a 75W-90 GL5 oil in there. If you think about it, you have a road train pulling 5 fully loaded trailers, in the outback heat, and under huge shock loads. The manufacturer specifies a 75W-90 oil for most differentials. Ambient temperature or operating temperature doesn't seem to be relevant.

Yeah I dunno, Harley box gets pretty warm but I've never tried to monitor it, I imagine it would get as hot as the engine case it's bolted to.