I think you've hit a bulls eye on the marketing strategy adopted for these oils. Even the M1 product engineer I mentioned earlier told me there wasn't a meaningful difference between the two, and that an air-cooled MC engine didn't have any exceptional qualities that necessitated a special oil. For my money, the cheaper 15w50 is a better choice because it has double the amount of moly compared to the VT variant, even if zinc and phosphorus are a tad lower.

 

I think HD and many other OEMs are leaning toward standardization of oils and recommending engine oil in trannies is to simplify oil-change procedure and to reduce the risk of confusion. I've never heard of it happening, but I'm sure someone, somewhere has unwittingly poured gear oil in their engine, which puts HD in the bind of warranting screw-ups by both riders and techs. Regardless, it will take some doing to convince me that a gearbox should run anything except gear oil, and I've never heard a good argument for using engine oil in a tranny.

On a recent thread someone mentioned that additives in gear oil are corrosive to "yellow metals" like bronze, copper, and brass--but I'm not aware of any such materials in a Harley tranny. I think some of the older trannies had copper-based shift forks, but I don't know if they are being used today. Anyway, almost all HD dealers sell some sort of gear oil even if HD isn't currently marketing any. Mine has been selling Bel-Ray in 75w90 and 85w140 for as long as I can remember, and another local dealer sells Redline Shockproof Heavy. If gear oil was a problem they wouldn't be selling it and HD would certainly warn against it if it was causing any problems.

In '07 before IDS was introduced I talked with HD Tier 2 support more than once about tranny noise, and he suggested a heavy gear oil to suppress the noise. It helped a little, but even though IDS cured the problem I never went back to the lighter 75w90 gear oil I was using before, preferring the heavier stuff for the smoother and quieter shifting it afforded.

 

Which brings to memory something that happened to me long ago. I bought a '60 Chevy pickup off a farm. The rear end was singing loudly, like it was almost shot. I put some STP oil treatment in there and it got totally quiet. I drove that thing for years and years with the same rear end. I don't know what's in that STP, but it worked in that case: I had nothing to lose.

 

Ring and pinion gears had sharp edges. The STP took up the slack, so to speak.

 

I thought this wasn't going to turn into another oil thread Been using Napa 20/50 in my bikes for years. $8/quart motor oils? Are you kidding me? Are you getting 20K miles between oil changes? These old V-twins are like a Briggs and Stratton mower engine, they last and last with basic maintenance. Oh, and I use Napa/Wix oil filters too, for a couple bucks each. I do splurge and use Chevron gas for a few cents more a gallon though Mainly because it's 3 blocks from my house.......

 

Motorcycle engine oil is formulated like a light gear oil, like automotive engine oil used to in the old days. That's why it works in motorcycle transmissions. Nobody here has mentioned unit construction engines like Jap bikes have. Their transmissions and clutches use the same oil as the 180HP engine does, and they seldom fail.

 

I don't think it's an issue of failure; more one of longevity. You make a good point though.

 

Yup.. NAPA 15W-50 full synthetic for $3.19/qt on sale

 

This couldnt be further from the truth.
On another note, I remember seeing the old air cooled Volks Wagons that use to cruise around in our parts, and in summer, temps were above 110F in some places so I wonder what oil they use to use back then until full synthetics became available, mineral I bet. I still see a few of the VW's cruising around now and then.
I also went down to local garage to get some more M1 and just found out that its now unavailable or relabeled, in Australia it seems anyway. Couldnt believe it after all this. A few of you are using valvoline, which one? I can get valvoline Synpower in 10w-50 which seems to be the goods. Meets or exceeds all the specs API SL,SM & GF4

 

You say couldn't be further from the truth .... but then you give no explaination ... what are you trying to say?