Sounds like your oil pressure sender is weak, or possible the gauge in the dash.
Are any other gauges reading funny? If so could be a ground problem.

 

If you were to go for an autometer gauge it should be around $90 CAN and then like 40 for a pod, i have no idea about the stock one though

You can see the pic of my pod @
www.portugesethug.homestead.com/jim.html



When i first start it up it will be around 60 but then drop to about 20 later on and when accelerating, i have never had it past 65 i belive

 

Since your planning on selling it, I’d check with your local Mazda dealer for a price on the sensor, may be cheaper than JIMMY54’s option. I’d double check the connector on the sensor; you could just have years of crud in it that are causing it to read a bit low.

What are you going to be asking for the car?

 

my guage was doing the same thing, the old mechanic said the sender was ok, but the guage was bad so i didn't really do anything about it. about a year later (just a few months ago) i had an oil leak at the front cover. when the shop took the cover off to fix it they noticed that an 'o' ring on the main shaft (something like that) had blown and oil wasn't getting to all of the engine. they said i was a lucky bastard and didn't know why the engine didn't go. $2 later for the 'o' ring and my guage works fine again!! might not be the same prob for you but it might be worth checking!!