I was recently given a DeVillbiss compressor from a Dentist's office.
It's a beautiful, quiet machine with 2 cylinders and an automatic dump
valve.

I changed the oil, and the dipstick has me confused. Does one screw
the dipstick all the way in, or just till the threads meet (as in
"dip" stick) to get an accurate reading?

Also, the copper pipe from the head to the tank is about 10 mm in
dia., does the usual loop thing in front of the pulley fan. This pipe
gets extremely hot. Would a larger diameter pipe looped in a similar
way reduce the the heat (and the moisture entering the tank)?

Many Thanks.

The dipstick doesn't need to be screwed in to take a measurement.
Either way you're well within the allowable limits on oil.
The copper pipe is a heat exchanger to dissipate the BTU's developed in
the compression cycle. The factory design is perfectly adequate. If it
ain't broke, don't fix it.
Bugs