The oil light is not a low oil level indicator, it's a low oil pressure indicator, you could have a sump full of oil and still not have the required pressure.
I would suggest you NOT drive the vehicle until the problem is resolved, if there is a low oil pressure issue, you could end up with significant engine damage. The correct way to deal with the matter is to fit a pressure gauge and check the pressure, however, it's not always the easiest way, so my next suggestion is to get a replacement switch and change it, see if the light goes out the way it should. If it doesn't, then you get a gauge and fit it.
The oil pressure light turns on at around 2~4 psi, normal pressure at idle with the engine warm is usually over 20 psi, and the FSM recommendations would be somewhere in the vicinity of 50~60psi at 3~4000 rpm - sorry I don't have the manual nearby, but those will give you an idea of what you should be seeing.
Possible causes of low oil pressure would be a defective pump or blocked pickup or excessive bearing clearances - if I'm not mistaken the filter is after the oil pressure switch, so a blocked filter will not cause a low oil pressure problem.
I stand corrected - I went to the manual and checked it - the oil pressure is in fact downstream from the filter, a clogged filter can restrict the flow of oil and cause the pressure to be lower than it should.
There are two possible scenarios there - one is that the oil pressure switch fails only when hot, I have seen this with aftermarket replacements, and the second is that there is a genuine low pressure condition because of worn bearings and as the engine warms, the oil thins out and flows more freely, causing the pressure to drop.
Idle oil pressure with the engine cold would/should be in the region of 60~70 psi - essentially whatever pressure the relief valve opens at - but warm it will drop to something in the region of 20~30 psi.
Were it my vehicle I'd get a gauge if only for the peace of mind ...
Actually - and I know this is off topic, but still pertinent - I see the oil pressure as being sufficiently important that all of my vehicles get fitted with an oil pressure gauge sometime during the first year of ownership - before problems develop.
What oil are you running? Nothing thinner than 5W-30 I hope.
Determining your actual pressure from an affixed / known good gauge is what you need to do for further troubleshooting.
Should your oil type / viscosity be correct and gauge readings actually show low, you can mask the problem with a higher viscosity oil or oil additive, but that's only a band aid fix. The engine is tired or your regulating valve is crapped up. And an OE filter change as suggested earlier is certainly a place to start.
Also, if your idle speed is lower than recommended (proly around 850 RPM for you) THAT will lower oil pressure to some degree.
How many miles on the engine and what oil change / maintenance periodicity has this vehicle had?
'99 Grand Vitara JLX, 2.5L V-6, 4WD, 5 spd std, '00 "Limited" leather interior.
'03 Honda Odyssey
'72 Chevy Nova. One owner, SB, A/C, power disc/drum & steering.
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