Just recently my P reg, Golf mk3 1.4 has started showing the oil pressure light on the dash. The weird thing is though, the light doesn't come on while the car is cold but when it warms up (around the middle of the temp dial) the light starts to come on at around tick over and doesn't go off until 2,000rpm sometimes even 2,500rpm

I thought it was something to do with low oil so I checked that and saw that it had the correct amount

I took it back to the garage I got it from and they kept it for 2 days saying that they changed the oil and replaced the two switches (I don't know for sure but I guess oil pressure switches?) After that they said they couldn't find anything wrong with it and gave it me back, didn't cost me anything. Told me not to worry and keep an eye on my oil.
Today I took it to my dad who is a mechanic but isn't that sure what it could be so he asked someone at work that suggested a thicker oil and an oil flush, as apparently old Golf's have trouble with low oil pressure at lower revs. So after doing that and removing a very thin oil out the engine the light now only comes on when the car is warm at tick over and goes off around 1,200rpm but I did notice when driving it home earlier that it sometimes didn't go off until just under 2,000rpm so I'm not sure that really did anything at all.
It's maybe nothing to worry about but it's driving me insane! It's obviously coming on for a reason

any suggestions?