Still does not sound like its fixed really. I think its either a warped clutch spring or friction plate in the clutch. Its not a major job to pull the primary and have a little bo peep.
My Street Glide has 56k on it now with the std clutch except for the heavy duty spring. It started slipping under load at about 35k but the spring fixed it. The clutches are generally very good but remotely possible it has been slipping for a while causing it to heat up and damage itself. A teardown will reveal all
PS Have you tried it to see if the clutch is slipping? Put it into a higher gear say going up a hill stay at a constant throttle and give the clutch a "fan"if the revs increase and speed does not.......
Would read nothing into the tyre life other than these big jiggers are very critical with tyre pressure settings and that can make a huge difference to mileage . I run my rear at 42 psi cold. All the heavy spring does is force the plated harder together so if you are not getting slip, well that decision is yours. Just to clarify about a heavy duty clutch, i am not suggesting a complete racing clutch with basket i am talking plates only. There are many brands out there with S/eagle and Barnett getting plenty of mentions.
CV, like you I'm an old fart with a wealth of bike building experience. my bikes have a few GP titles, every Guinness distance jumping record and too many National titles to count. I'm new to HDs though and keen to learn.
Re: the OP. I'm 99% sure the issue here is interaction between the two lubes. To test this will cost you $30 or so. Warm up the primary to full temp, tip in 1/2 bottle of Wynns Engine Flush and ride the bike up and down the box. Don't use any other brand except Wynns, it's the only one with an EP rating equal to the original-fill oil. Dump the hot oil/flush mix and refill. If a partial fix is evident, pop the clutch plates out and thoroughly clean all the frictions with Shellite.
Given that your issue happened instantaneously upon changing the oil, it's not depleted oil, nor a mech issue.
Worth a try, as is resetting the clutch ramp system to spec.
What synthetic oil did you use??
Thanks but it doesn't quite work that way. I'm wired differently to most which creates problems. I'm whats called a high functioning autistic. It means I know a whole bunch of shit I can't possibly know without degrees and stuff that I don't have. My list of technological achievements is pretty long. It doesn't mean shit to me as I don't see the significance. Means I have had a lifetime of anxiety and depression. I don't feel emotion like normal people and my reality is quite different It is what it is.
On the other side I am 62, I love drinking too much piss, fucking hot and nasty women, outrunning the cops(I even run a flip plate on my ultra), generally being a deviate and crazy bastard with a fuck you attitude but also like to help anyone in need.
That just means you won't stand out too much in this place.