I have been following the excellent advice from this forum and have isolated my problem. When cold my 420 has 60 lbs compression in #5 when at operating temp its 135 lbs,#6 140# at all temps all the rest are 150lbs. The car has 77000 orig miles and I love it. Question, What are the possible causes of the variance in compression?

My 2 cents worth
Could be crud in the piston grooves where the rings sit. As the engine warms up and metal expands rings could pop free. Maybe another member can suggest what to do to clean if it is the problem. Only other thing I can think of is a valve is not seating properly. Is this car mainly city stop and go driving - or is it highway driving? Was the car sitting for a long time before you started driving it? at only 77,000 miles it sounds like it has had long sit times. My 560sel, when I got it 3 years ago was sitting for almost 2 years. It burned 1 quart oil about 500 miles, if I remember properly. I changed the oil and added a quart of Duralube. - 1000 miles later after a highway vacation - the oil burning stopped - the oil rings broke free. Maybe your compression rings are sticking on that cylinder??? Just a thought

MERV;
Take your car to a reputable shop and have the BASE setting on your lifters checked and / or replace the lifters on the low cyl.
M.F.