I completed my annual inspection a few days ago
New engine oil
New 50/50 antifreeze/water
New drive shaft and ball bearings on cog belt PSRU. These PSRU parts I
damaged myself when dismounting from the engine.
I have not flown the aircraft yet since inspection, prolounged ground
running failed to raise the engine oil and antifreeze temperatures above
150F-155F. OAT about 32F to 35F. These engine temperatures are not
standard for my 1986 engine, which would easily  reach 180F after a few
minutes of ground running at 3000 rpm.
A trusted friend has suggested that my old coolant was much too rich in
antifreeze due to the water content having evaporated over time, but
please advise
Yvon

That is possible. You should change out the anti freeze.
Should be about 50 50.

It is running cool because the OAT is 35 F.
One degree F drop in OAT is 1% better cooling.

Paul Lamar