Well I decided to do a little oil study of my own after reading all this. I really don't have a dog in this fight so I decided to just run an observation period and note what I saw.
My background: I went to school to be an aerospace engineer and ended up in the computer field as a software engineer. That's probably a whole other study tho
It does give me a reasonably methodical outlook, and a healthy respect for observed effects rather than just theory. I'm a reasonably experienced amateur mechanic who has restored several cars, bikes, snowmobiles, boats, and airplanes. I am not Mr. Goodwrench by any stretch.
I just wrote down my observations about how the bike acted/sounded with various oils. This is no more scientific than that - I recorded my observations of oil performance in a CB750 K4. Take it for what it's worth. I always take notes on how my classics are acting in general, so the parts that are particular to recording oil performance alone are just the last 2-3 changes. I did not know I would be looking at oil closely until the last 3 weeks.
My bike's background:
It came to me after 15 years of sitting on it's center stand. It had some mystery oil of unknown age in it. I changed it immediately for some Kaw 10-40 dino just to get it home.
1) My first real oil for it (1 week later) was Kaw 20-50 dino. Why do I keep using Kawasaki oil? Because my nearest LMS is a Kawasaki dealer
I went with 20-50 because I'm used to running classics and it's generally thot that bumping up the weight isn't a bad idea as seals etc age. That oil was fine, no leaks etc.
2) I decided to change again after about 2 weeks. I had been running Sea Foam, Marvel, etc to clear out any crap from the long storage. Again, I went with Kaw 20w-50 but this time synthetic. I mostly wanted to get a brand new filter on, as I hoped the Foam etc had loosened some crap.
At this point I begin looking for other things from my oil besides just being clean. Those are:
a) No leaks. The age old enemy of all classic vehicles.
b) Quietness of valvetrain and gearbox.
c) Shift ease.
d) Neutral hunting.
During the previous period I had been wrenching all over the bike just to get it running so everything was changing and any observations on oil qualities would have been meaningless.
I ran that 20w-50 synth all the rest of last season and was content. Going into this season tho 2 things caused me to hunt for a new oil: leaks started and neutral hunting. By neutral hunting I mean that after a long ride when I would go to put the bike in neutral I would have to hunt for it a bit. The detent up from 1st would be hard to pull thru and coming down from 2nd would be to easy. So I'd end up "fiddling" with my foot for a few seconds and it really annoyed me. Also the engineer in me said that heat was effecting the gearbox parts enough to make them "picky". Less heat would then equal less snippy-ness I thot. Also I began to blame the leaking on the age old rumor of synthetic molecules being "extra small" and going thru seals dino wouldn't.
3) So I decided to use what the manual actually called for and go with good old Honda 10w-40, although I used synth. What I got was more gearbox noise and no decrease in hunting or leaks - in fact more leaks now from the right as well as left side of bike.
I've been reading oil threads/fights on this forum in the meantime and thinking of recording oil specific obs.
4) "Yikes it's the synth!" says I. I switch it out for Honda 20w-50 dino. more gear noise, more hunting, slower leaks. The slow down in leaks was not dramatic tho, and IMHO totally related to the thicker oil. I'm pissed: "I've hosed my seals" groans I, while ordering a $100 gasket and seal kit.
At this point I begin looking for good old mechanical instead of chemical solutions to the leak problem and I find them - by perusing some other threads here and poking my nose around the bike. The stupid chain oiler has been causing all the leaks on the left side. I turn it completely off. The really big leak on the right side is even dumber: the oil tank drain plug seal. The big washer on that has some big scratches on it and it is letting oil pass. A new washer and the addition of a soft washer stops that cold.
The bike stays dry for a week while the gasket kit collects dust in my house. As a long time classic owner I am just totally averse to breaking any old seal or gasket that is holding fluid back (they're too rare and precious), so I will simply NOT use any of the new gasket kit until some new leak happens. The bike is dry to this day.
5) All this talk of Rotella was interesting and wanting to give my new seal fixes the ultimate challenge I decided to go with the really light Rotella T synth: 5w-40. A week later I still have no leaks and I'm a bit surprised. My neutral hunting is much better and my gear noise and valve train is slightly better too. I would not have expected
all those things to get better with the lighter oil.
For all my target goals the Rotella T is the best oil
I have found in
my bike. I'm excluding leaks from my goals here, because I found purely mechanical fixes for them. I did not expect the lighter oil to quiet the bike but it has. Shifting effort in general and neutral finding in particular are much better.
I have always believed that synthetics are better oils. The question is a bit more complicated for classic owners tho. I do believe that synthetics react differently with antique seals and clearances. If the classic in question will hold the synthetic without leaks, and if it doesn't burn it by passing the rings etc, then I say run it and feel good about it.
I am a bit concerned by the Rotella's mighty light 5 weight tho. If I was still in Florida I wouldn't run that but Minnesota is so much cooler I'm going to give it a shot. I'd prefer a 10W-40 synth tho. Shell only offers that in dino in stores near me.
So that's what I saw, for what it's worth.