MG Car Repair: Oil pressure on gauge drops when rpm increases, oil pressure gauge, defective gauge

Question
Out driving my MG today I glanced at the oil pressure gauge that generally reads from 35-50 lb and saw that it had dropped to 10-20. On increasing the rpm it actually fell from 20 at idle. Water heat was OK. Oil at recently been changed so I knew that was OK.  But the car sounded fine...no untoward noises so I did drive home (about a half mile).  Checked oil level and was fine.  So do you think this is a defective gauge or pump?  Should I even think about starting it again!

Thanks
JT

Answer
Hi Joe.  This not an easy question to answer.  A sudden drop in oil pressure is something that should not be ignored and you are correct to be concerned about it.  You don't say what MG you have, but this is such a general question that I can give an answer applicable to most MGs up to 1980.  

I notice you have recently changed the oil.  Have you used the correct oil?  Modern semi-synthetic oils are too thin and they can be harmful to a classic car.  If you have used a 10W/40 or thinner, then that could be your problem.  You need to use a 20W/50 grade mineral oil.

Is your engine in good condition?  If it is old and tired, then maybe it needs refreshing with new bearings on the crankshaft and perhaps a new rocker shaft and new rocker bushes.  Before you do that, check the oil pressure relief valve.  

If it is an MGB or a Midget, then maybe the oil pressure relief valve is worn and leaking.  It is only a ball bearing.  Undo the access plug at the side of the engine and pull the spring out.  If the ball doesn't fall out, use a small magnet on a stick to extract it.  Is the ball circular, or does it have a ridge of wear on it?  If worn, fit a new one.  Put the ball in the hole, insert a steel rod and give it a single smart blow with a mallet to seat it.  That could easily be your problem.

One other item worth checking is the flexible oil pressure pipe near the distributor.  They do sometimes start to leak, producing a low reading at the gauge.  If yours is leaking, fit a new one.

It is most unlikely that the oil pump would suddenly wear out, and gauges do not suddenly read low.

Please let me know when you find the problem.  In the meantime I would not recommend that you use the car.