Thanks for all the comments.

I did 2 things
1) changed engine oil to Rotella 15W40 , dropped the oil pan before and checked the screen (changed it actually)...
Was clean like a whistle down there.
Since then the pressure doesn't drop below 1bar...mostly 1.5bar at idle.

2) I flushed the bad coolant (citrus) and filled with G05. Drained through radiator and block drains. While I was there I changed the thermostat.
Temperature now goes slightly above 80C.
So far no engine oil is showing up in the coolant tank...but I have only 500 miles on the new coolant thus far.

I guess if engine oil pops up I'd try and tighten head bolts slightly. What do you think?

I have used seafoam in the tank and added some ATF.
Since one week the erratic movement of the gas gauge stopped! I'll see if it starts again.

Thanks so much for all your support.

One question regarding the transmission oil change and downshift behavior of the trans....
ATF: As I don't know how old that stuff is ...Would you just drain ATF and fill up with new fluid (without changing the filter)? At least then I know that fresh fluid is in.

Downshift: The car does not downshift unless I really push the pedal to the floor- WOT-switch..it works) How can I adjust the cable properly (from throttle linkage down to the trans) and does the adjustment influences shift points for upshift (I guess yes) and downshifts?.
I think the car drives great but what I miss is a more agile shift (like on a BMW).

How much work is it to change spark plugs and service ignition? Anything to adjust or is the ignition still on distributor?

Best, Martin