I recorded 5609 RPM in a single 1 second reading with slightly longer bursts >5500 while taking an afternoon drive from sea level to 4000 feet with 0 EV after forgetting to charge the night before. I was intentionally pushing it, accelerating hard up the steepest parts, trying to see if I could detect any power loss or "angry bees". While the engine was loud for those short bursts, it sounded more "busy" than "angry", and I didn't notice any real power loss (the power bar did seem to peak out a section before the max, but I was still able to gain speed uphill). I agree that the engine isn't revving that high or under undue stress.
We drive the Clarity about 22K miles / year. Today I took the car to Jiffy Lube and told them to set me up with a 10K mile interval which should occur about twice a year. I suspect the ICE will have run during about 2,000 of those miles so that is way overkill but reasonable.
What is this "oil" of which you speak? I looked under the hood of my Vortex Blue Clarity, and could not find anything to do with "oil" - no filter, drain plug or anything. I think mine also might be defective since it has no gas cap or any way to add fuel from the gas station. Should I take it back to the dealer? A dealer did offer to change the oil when I brought it in for another issue. Tempting to tell them to "go ahead", but pointed to the badge on the side and the kid said "Oh. Guess we can't do that". He was a bit sharper than most of the service greeters I've dealt with. Carry on. I still enjoy a good oil debate/thread, even if it is purely vicarious!
Well, 4000 ft is not really that high and not much different than sea level. Try 8,000 ft. I drove trough Monitor Pass (8,314 ft) with 2/3 battery, but in HV mode. The engine was bursting it seams at 35 mph. Even relatively flat roads above 7,000 ft strains the engine above 70 mph. I like to have a third driving mode, where the car blends engine and battery charge.
I keep tab of how many gallons of gas I use at Fuelly.com to get an idea of miles on ICE, like others, a once a year change is all I need.