SO WHATS THE PEOBLEM?:
The oil pressure warning light is bought on by the oil pressure switch which is attached to the engine. On turning the ignition switch on,you should always check that this light comes on.
THIS THEN TELLS YOU THAT:
1.The oil pressure switch is working,
and
2.That the warning light bulb is serviceable.
After you start the engine. This oil pressure warning light should go out. This then tells you:-
1.That the switch is working.
2.That the circuit is seniceable
and
3.That you have both sufficient oil and oil pressure. Should you get VERY low on oil, then this light will come on to tell you.
SO! if this light comes on at low engine rev's, first check that you have sufficient oil by using the dipstick. Then the next step is to establish wether you have low oil pressure or a suspect switch. This switch costs less than a fiver and takes about 30 minutes to change(including washing and tea drinking time!). The next step is to fit an oil pressure gauge in place of the switch. If the readings are low then it is the oil pump that needs the attention not the engine. The oil pump is replaceable,some are cheap(£15)some are quoted at £100 plus!so think before replacing same.Remove the end plate from the pump,and check that the gravity feed pipe from the sump is clear.if not,clean it out!if the whole oil system looks gooed up,then consider running the engine for 10 minutes with flushing oil,then change the filter, and replenish with clean engine oil.
NOTE:if you have to top up the oil on a regular basis,and you don't have a black pud(on the drive or the garage floor to step in). Then perhaps you are burning it! Don't worry, a ltr of oil every 1,000 miles is far cheaper than an engine overhaul!if you are not happy, then you can always have a compression check done on the cylinders.But that is another story!

FINALLY THE MAGIC FIGURES GIVEN BY VW:
OIL PRESSURES.
T2.oil light out-2to6.4 psi.
Minimum at 2.500rpm. single carb-42psi.Twin carb-28psi.
T3.Air and water cooled plus diesel.oil light out-2 to 6.5psi.
Minimum at 2.000rpm-29 psi.