Tuned Perfection Air-cooled VW Performance

Without pressure in an engine you can't have flow, and without flow you can't cool or lubricate. If you can't generate pressure then your bearings/rings, journals, etc, are worn out. Pressure is a measurement of resistance to flow, but you must have it in order to move oil through the system. No pressure or extremely low pressure and your not likely to be moving much oil through the bearings. Which results in higher temps, and eventually ceasing the engine (or worst) due to lack of lubrication. Without pressure you can not create a hydrodynamic wedge between bearing and journal witch in turn helps to increase the sheer strength of the oil film, and that's what protects the bearings surface to begin with.
The real question here is what is needed pressure vs to much pressure. No doubt you don't want more pressure than is needed, but based on an engines expectations, and driving habits that can become a variable at that point.

Ideally in a air cooled type 1 with the oil at 200* I like to see 15-30 psi at idle and 45-60psi full in by 5000rpm. The variable is based on oil viscosity and engine condition. Any less than 15psi at idle and I go hunting for the problem. I doubt that its a "tune-up" related issue. I would check out the oil pressure relief springs. There are two on your engine case, each having different functions, but none the less, remove them both, and check the pistons. Sometimes they stick, and cause erratic oil pressure related issues.

However, Oil that is heavier can drive UP the operating temps, first by requiring more power to pump (because the oil is thick) and then by overcoming the valve that is designed to bypass the cooler when the oil is cold. And when the oil gets too hot the pressure drops. I would think that some key clearances change with temperature as well. The thicker the oil, the higher the temps and the lower the pressure

This would explain your situation somewhat. At highways speeds, the engine is working at higher RPMS, thus the temps are higher, thus the pressure would be lower! At idle, its the opposite. Lower temps, higher pressure. But of course, with a heavy oil. try changing to a 5w30 semi/ful synthetic and also check/clean those two pistons. I have a feeling after you switch the oil and clean those pistsons, you'll be fine!

I had a similar problem a while back. During a highway drive, the pressure would drop down to idle levels for a while, and back up again to normal levels. Changed the oil to 5w30 synthetic, and she's was good to go. I did also change those pistons/springs in an earlier tune-up as well.

Update us on your findings! Best of luck!

Marc, that is kind of low. Should be 10psi for each 1000rpm as a rule of thumb. The weight of oil seems fine. I am a bit concerned with the cooling of the engine. Oil temp seem high for a stock 1600. I had mine at 210 with the turbo 2387!!! Did you get the carb issues sorted out yet?