When full, the oil pan of a 91 takes only 3.5 quarts of oil.  "Only a
quart low" means 30% less oil in the oil pan.  That's a significant
amount.  I find that on the track, a Miata needs to be filled full and
a bit more to prevent oil starvation.

Gus, when your engine is turning at "freeway speeds", there is over 1/2
of your oil in a spray condition inside your engine. It takes time for
the oil to run back down to the oil pick-up to be pumped again. Don't
drive your car fast, with a low oil reading ! Remember the Miata also
has a "spray oil system" to the piston and cyl, assy's that most other
cars do not have. It will not harm the engine to run with 4 qt's of oil
when you are pushing the engine hard. ( autocross or freeway cross
country ) AND you will find that up to 60% of your engine heat is acting
on the engine oil. ( oil coolers on 94 and on. ) are not there just
because they didn't have anything else to do with them. :-)  

     Bruce     RED    '91

Slightly off topic, but I've noticed that my miata has a slight oil
pressure drop lately. Not severe like this, but running about 20% lower
than normal.
Usually, 3,000 rpm oil pressure is about 45 psi. Lately it's running 35
psi or so, just to the right of the 30 mark. Right now I'm not too
concerned, as the oil pressure is high enough, but I'm expecting it to
get worse. Change oil/filter doesn't help.
I normally run mazda filters and a major brand of dino oil.
Idle pressure is the same, or close enough that I can't tell the
difference.
Start-up pressure is max 45, used to be 65-70 psi.

What are your thoughts as to where to start looking.