Yeah, I'm kind wondering now if this might be a coolant issue.
But, I was figuring for sure the oil analysis would have disclosed that.
I have had a couple of cases where a leaking head gasket was letting water track from a coolant passage over to an oil drain passage. But not on Toyota engines.
The engines had good compression and did not show any of the usual signs of coolant leaking into a combustion chamber.
One engine that bugged us at a job I worked back in the 70's led us to drain the oil out of the engine and place a catch pan under the open oil pan drain.
Then we pressurized the cooling system and left it.
Yes, pressure leaked off and we pumped it back up.
In a little while we started to see coolant dripping out the oil pan drain. We also had all the spark plugs out and none of the cylinders were filling up with coolant.
Pulled the head and finally saw where the coolant had etched its way from a coolant passage to an oil return passage.