I went for a drive today in my '66 coupe and i noticed the following:

When the engine is cold i see the oil pressure gauge inclining to the middle of L and H, but when i drive the car for 10 minutes i notice that the oil pressure needle goes down again (back to L). So I am guessing i am having a loss of oil pressure somehere.

What are the most common reasons for this to happen ? Or is this normal ?
I would guess my oil pump is malfunctioning... correct ?

 

Michael, the gauges on these old cars are bad at best. If you don't hear a ticking from your engine, then your oil pressure is just fine. If you have any reading at all at the oil pressure gauge, then your pump is working. Relax.

 

Well i do hear it ticking sometimes after i turn the engine off. The radiator is also making some noise.
I thought it was normal for an engine when it cools down ...

 

It's ok to tick when it's cut off, thats just the metal cooling off, so it's normal.

 

I always thought that clicking was the closed valves popping up as the heat is released from the chambers.

 

To be safe you may want to get an aftermarket oil pressure gauge, It could be mains, cam bearings, or something else... Oil thins out as it gets warmer, doesn't it? I noticed on my Durango that the needle will be as high as it can go (opening the pressure relief valve on the oil pump) when it's cold, or has new oil in it, but then after it gets warmed up it tends to be at about 1/3 the way through.

 

Go to your neighborhood auto parts store and invest $20 in a mechanical gauge. It'll mount under the dash and give you an accurate reading. I had to do the same thing...

 

The mechanical guage may be best.

He should at least replace the oil sender unit. That will be the easiest to install.