[KTL]

Just because there's a free discharge to the oil tank, don't assume there's little pressure there. And the discharge to the tank isn't truly free. In the 73 to 89 tank, the oil goes thru the filter before it is dumped into the tank to re-feed the engine. But prior to the filter, there's a pretty significant obstruction by that thermostat. If you look at how the oil goes though the t-stat, going thru the temperature regulating unit's windows is not all that smooth. After the regulator, the passage to the return is pretty tight too.

So that's where I think the restrictions are. Plus cold thick oil is a big deal too. Note the thread I linked where Mike Bruns said that was a big issue on the one setup he observed