I've never ridden or owned one, but the Yamaha Warrior is very similar to the HDs. Air-cooled V-twin, pushrod actuation, fuel injected, similar power, etc. I don't suspect anyone runs a 60 grade in them, but like I said, I have little knowledge about that specific bike. What does Yamaha suggest as an oil grade? I would gather it might be a 10w-40? I'm not telling; I'm aking because I don't know. But it would be interesting as a fair comparison. This would be another plausible application for 15w-40 HDEO?
Currently, I'm using up my old CI-4+ Rotella in my GL1800. I've typically used 40 grade oil. Recently, I've purchased a large amount of Rotella 10w-30 HDEO gallons for future use in my Kubota, my Goldwing, my air-cooled lawn equipment, and my 1966 Mustang 289 V-8, etc. They all spec 30 grade as applicable, so I'm going to try it next.
My buddy and I are hard core riders. We start out on a weekend morning, and return that night. Often 400+ miles in a day; all backroads and state highways for the scenic routes. I'm not a bar-hopper, in that regard. Don't like riding in large groups (too many potty breaks and constant fuel-stops). We like to ride long and hard. That's probably why we've had such good luck with HDEO. Engines might get warm, but they stay cooled by (relative) constant motion. Occasionally we'll ride to a race (Indy 500, Brickyard, etc) or a car show, for the ease of parking, etc. But overall, our bikes stay moving. So super-hot idling doesn't happen often for us. So, the HDEO UOAs show little to nil difference when compared to the expensive POAs.
I'm sure that a lot of our experiences come from different riding styles, perhaps. And the OP (remember him?) should consider his riding preferences when selecting a lube. I think for most bikes, even air-cooled ones, HDEOs work perfectly well
if used with reasonable OCIs and a high amount of air cooling flow. To the contrary, if long idling and ultra-high heat are often experienced, then I'd agree that a higher grade might be a better choice. But I still ask if the cost is worth the results, looking at expense of motorcycle-only PAOs. Perhaps, if a higher grade is warranted, then a dino PCMO 20w-50 oil from a major brand such as Valvoline, Mobil, etc is a good choice, and just shorten the OCI a bit. Admittedly, I would never suggest a 30 grade in a Harley; there are limits to my phillosophy, after all.
Anyone choosing a lube should always consider several things when making the selection. Operating environment, severity of use, cost of product, availability of product, OCI length, ease of service, etc. They all play into the decision. All I'm saying to the OP in this case, is consider all the options; perhaps HDEO would fit the scenario, for a LOT less than MC-PAOs. And let the UOAs be his guide.
After all, isn't experimentation and sharing the results the ultimate joy of the BITOG experience?