oil pressure light

Hi
I have a 3.0 Ltr terrano sve 2005. The problem was the oil light coming on intermitently. Even if revved it still stayed on. Next time it was started it would be off and all was ok. This happened a couple of times. I suspected the oil light switch and got the garage to change it, still the same. Then i found out on the web that there are two oil pressure light switches fitted, one by the oil filter and one by the starter. Changed the one by the starter and now all seems ok. There is a word of warning though when the garage changed the one by the oil filter they fitted the wrong one. It would appear that they work at different pressures. When I tested them the one by the filter needs to operate at 0.14 bar 0.3 that's 2ps1-4psi. Where as the one by the starter seem to be set to operate at around 2.75 bar that's 37psi. I can't work out the logic behind this but it seems to work. ( if anyone knows why, please tell me). If the wrong switch is fitted at the oil filter then the warning light comes on when the engine is warm as the oil pressure should be about 1.5 bar- 21psi. Just for the record the pressure at 2000 rpm should be 5.5 bar- 77psi and 7 bar- 100 psi at 4000rpm. Hope this helps someone