Part of your problem maybe that it is slightly over the "Full" mark. That is not the "Full" mark. It should be labeled what it really is. It is "Maximum" do not ever exceed no matter what mark.  Seriously, the water boxer can be very sensitive to oil level in addition to viscosity. When overfilled the crank and cam will churn in it causing it to get hot and possibly even aerated. Both will reduce oil pressure. In addition the excess will get carried past the piston rings (pistons on their sides) and get carried out where it can foul the o2 sensor and the catalytic converter. This happens over time during extended high speed-load operation.  And what oil viscosity are you using? 

Check the archives for more information. Basically there are two oil pressure switches. The higher pressure switch is looked at when the engine is over 2,000 rpm. That will give the light and buzzer. Once on it will stay on until the oil pressure becomes adequate or you cycle the ignition switch. If you have the correct .8 bar sensor it is telling you that you didn’t have 11-12 psi oil pressure over the 2,000 rpm. 

Dennis

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of Kk Al
Sent: Monday, May 4, 2015 5:34 PM
To: vanagon@GERRY.VANAGON.COM
Subject: That oil buzzer

87 Syncro here
We are on long weekend. It's a nice 76 degrees out. Van is running well, temp is normal. Fresh oil and filter. When we pull off an exit the oil buzzer goes off and warning light turn on. We stop, check the oil. It's slightly over the full mark and clean.

I thought I read somewhere that the oil light can come on due to high or low pressure.

Thoughts?