I don't think that's correct -- I think the heavy varnish in the front head is due to the PCV valve. Gen 2 Odysseys put the PCV valve on the rear head, and on those the rear head gets heavily varnished. And anyway, I think the coolant flow is split evenly out of the thermostat, so they ought to be equal in temp unless VCM is running.
I do think you have a good maintenance plan on your Odyssey fleet and I'm interested in seeing how they hold up long term. I wonder if the PCV valve replacement might be the key to making them last.
Yes, the PCV is too small and inadequate. I replace every 30k miles.
I stocked up on them at an online Honda dealer before inflation hit, and have a nice stach even if part eventually gets discontinued by Honda.
Your right about the heavy varnish on the front heads.
I am purchasing HPL engine cleaner for the next 3 oil changes
on each van (1 quart HPL Engine Cleaner + 3.5 quarts regular full synthetic).
HPL has a good deal if you buy the 12 pack.of the Engine Cleaner.
I am going to try to keep all 4 Odyssey's another 12 years.
I just don't like the newer vehicles styling, etc.
It's so nice to own 4 vehicles with no oil consumption
and no oil leaks. Also I paid cash for them, and have no debt,
so am likely to hold onto them for as long as possible.
I do a drain and fill of the ATF every 15k/3 years, whichever comes first,
as many owners on the odyclub forum say if you do that you'll likely never have any transmission issues ever. Keeping my fingers crossed.