I have a 21-yr old jag. The oil gauge indicates 50 when I apply the gas and while driving. However, when I stop at a sign or light, the gauge reads between 20 and 10, but jumps back up when I begin to move.
NOTES:
The oil level is great.
The oil pressure sending unit was replaced.
The oil pressure switch was replaced.
1. Should I be concerned? Is this going to damage my car?
2. Any ideas as to why this is happening?
3. Is this just because my car is old?
Any oil pressure questions should be answered by installing an external gauge and taking a reading from that.
It’s possible that depending on the mileage, the crankshaft bearings are getting worn.
Great. I will check into that.
Thank you for the advice.
I don’t know the correct oil pressure for your engine, but that sounds close to normal (I’m assume the gauge is in psi). Have you checked the owner’s manual for the correct values. For example, mine has a hot idle pressure of 1.5-2 bars (about 20-30 psi) and a running pressure greater than 3 bars (about 45 psi).
Has the oil pressure changed? If the pressure is now reading lower than it used to, I would check it out regardless of the values.
I believe it is psi.
Here is what the handbook states:
“…The minimum pressure at 3,000 rev/min, when hot, should not be less than 2.8 kg/cm2 (40lb/in2).”
It doesn’t say anything else other than that the gauge indicates the engine’s operating oil pressure…it does not indicate the quantity of oil in the sump. That’s it. 
Also…the oil pressure LIGHT never comes on.
Just the gauge indicates a drop in pressure in when I come to a stop while the engine is running.
Assuming the gauge is accurate, the change in pressures sounds completely normal. It will normally be higher at faster RPM, and will drop some when you stop the car (all once it’s at normal operating temperature).
I don’t know what the actual pressure specs are for your engine, but on the outside it looks normal to me.
10-20 PSI sounds pretty low for a shell bearing engine to me.
The oil light may not be coming on because with most sending units the light will turn off at a very low pressure; generally around 6-7 PSI, so the 10 PSI may be keeping it out.
I don’t know, if his book says 40 psi at 3000 rpm, 10-20 psi at idle sounds pretty reasonable. As I recall, the minimum idle pressure in my benz manual is 0.5 bar (about 7 psi), it actually never gets below about 1.5 bars (about 20 psi) in my cars. But, I don’t know very much about british cars.
The lower pressure is probably due to the engine bearings, but I don’t think that it means that they have to be replaced just yet. Its just normal for the oil pressure at idle to go down as the engine wears out, but you may have plenty of life left in it yet before you need to do an overhaul.
You may want to be very concerned about the oil you are using. Some of the new oils (SM) are not good for these old flat tappet engines and could cause you some serious wear. Consider using a 15w40 diesel grade oil like Rotella or Delo 400.
What is your oil weight? I’d use 10W-30 minimum and more likely 10W-40. Oil pressure is normally expected to go down a little with a warmed engine at idle. The new SM oils should barely be OK with a broken in flat tappet engine but with a new hot rodded engine with flat tappet cam followers, beware! I will add a quart of diesel oil for our older engines. Modern engines have roller cam followers and can safely use SM oil.