My two cents. Over 25 years of running Hondamatics. Engineering background. I've tested just about everything that's out there on my Hondamatic 4 cylinder. Not quite what you're looking for on the street as I bracket race Hondamatics. I've done a lot of A-B-A testing in similar weather conditions (density altitude) checking ET and MPH. If an oil is better, your MPH usually improves. If it's worse, just the opposite. I've tried single weight, multiple weights, motorcycle oils, and car oils.
Basically, synthetic oils and synthetic blends are too slippery. The torque converter is a fluid coupling. The slicker the oil, the greater the slippage through the torque converter. Your power is reduced somewhat. The ability to handle greater heat/oil temperatures may be an offset for street running. Your gas mileage should also suffer a little with synthetics slipping more.
High viscosity single weight oils have a very narrow range of oil temperature that the bike would like. Again, not what you would want on the street. I ended up narrowing it down to multi-weight mineral based oils. Not a huge difference between brands. I got the best numbers with a 20W-40 oil that is no longer produced. I'm from New England, so I don't see a lot of very hot weather. Worked better than 0W-X, 5W-X, or 10W-X oils.(X could be 20, 30, or 40) Very small differences that cost might influence.
Heat becomes a very important issue for a Hondamatic. (especially if you ride in a hot temperature area.) Mineral based oils can take just so much heat before you'll get a viscosity change. Basically you make the oil a lot thinner. Once that happens, the bike slows down as the oil slips more going through the torque converter. The oil basically becomes junk. Changing the oil is the only way to get the bike running "normally" again. I run an aftermarket external oil cooler until the air temperatures get below 50 degrees F. When it cools off as my season ends, I'll block off the airflow through the external oil cooler.
There are expensive racing synthetics that work better the hotter the oil gets. The price and availability makes it not a street option. Bill Benton could help you with more information on the exotic stuff.
Everybody has opinions on this subject. Very few have done proper testing with numbers to back up their opinions. The best oil for you is the one that you convince yourself works best. I'm just offering my opinions of the dozens of oils that I've tried to get the maximum out of my Hondamatic.
Oil additives for new engine break ins and other special functions I haven't tested for. Let others offer their opinions for that stuff.
