What's my oil pressure guage telling me?

Factual Questions
1

So what? My oil pressure is a bit low. When I’m at highway speeds, it’s just fine - it only drops when I’m at idle at a light (at least, so says my guage).

The the question arises - what is my oil pressure all about?

Do I have an oil pump? Where’s this pressure being contained, in the crankcase? What’s it used for?

This is engine oil, right, not transmission fluid being indicated by the guage?

My idiot lights are all flickery once and a while - the A/T Oil temp, Battery & Brake will all flicker together, clickity-clickity-click. I suspect it’s a dashboard short not a problem but I thought I’d mention it.

I just had my valve cover gaskets replaced and I’m no longer leaving large puddles of oil everywhere I park. There’s some small drops still but nothing significant. The engine seems to be running fine, if not underpowered, but that’s the car & vintage showing up there.

Oh - 1990, 4Runner. 255,000 miles (engine “rebuilt” about 15K miles ago). I just had the oil & filter changed after I got the valve covers fixed up.

2

You have an oil pump. It’s located at the bottom of the engine. It sucks oil from the crankcase and pumps it to the various bearings (most notably, the crankshaft and camshaft bearings). The oil pressure is measured right at the outlet of the pump. It’s normally a bit lower at idle than at full chat.

3

Sounds like you’re either a bit low on oil (I used to have a Subaru whose oil pressure gauge would wag like a happy dog when at idle to indicate I was low) or the clearances in the engine have opened up to the point that you might want to consider going up a level in viscosity- say from 10w30 to 10w40.

4

Hmm. I’ve been runing 10W40 normally but they didn’t have it “on tap” at the oil-changing bay so I let them put 10W30 in. Might have been my mistake.

Next time I’ll pay the “from-the-bottle” prices.

I know I could change my own oil but it’s so cheap to get it done and such a dirty job.