Bear with me, I'll try to include as much pertinent info as possible.
1991 NA, purchased in December.


Engine stock, all emissions intact and functional, AC and PS (both work and are leak free).

Motor was supposedly rebuilt within past 10K miles (speedo shows 170K) but no supporting evidence/documentation.

Since buying the car I have-

-flushed fuel tank, new pump sock, new filter and fuel lines
-two oil changes, new filters (Castrol 20/50w oil)
-new plugs and wires
-new battery and FD alternator
-new belts and air filter
-new radiator (CSF, all metal), hoses, thermostat, cap and coolant (obviously), bellypan and fan shroud intact
-checked timing, adjusted TPS
-myriad other details not directly pertinent (I don't think...).

Anyway, car has always run fine, starts immediately (temps have ranged from -10 to 100 degrees since I've been driving her), AWS functions and steady
750RPM idle, NO 3800 RPM hesitation- pulls clean from idle to redline. NO hotstart/ flooding issues ever (knock on wood).
City mileage is consistent at @16MPG ( car is DD). Only one highway trip so far, averaged 22MPG for that 300 miles.

Since resealing the oilpan (don't ask) the underside of the car is completely dry- no leaks or drips.

Oil cooler has been flushed and the thermostat tested good- cooler gets warm across entire length. No leaks in the hoses.

Oil pressure has always read low (stock gauge, two different (used) sending units), @15PSI at idle and 40ish above 3k RPM. Installed SPI gauges (oil temp/pressure) on Mazdatrix pedestal adaptor and they read higher but I don't trust 'em- they react to voltage changes too much (I think).

Water temp (stock cluster gauge) never varies from halfway mark (come on Roen, figure out how to linearize these things!) and coolant level sensor has never come on (although I know it works since it buzzed like crazy when adding coolant to the new rad...).

No Mazda compression test but the "poor man's" test showed even, rhythmic pulses (good apex seals?) with a combined pressure of @80PSI. Mechanic described results as "Not great, not terrible- get ready for a rebuild".

Spark plugs (now with @2K miles since install) look normal according to my mechanic...no buildup and good even color.

Whew.

Now for the weirdness.

Oil consumption is all over the place- one week I added three quarts, the next week oil level didn't budge. No black smoke-ever- and no leaks either. I check the oil with the car parked in the same spot, so it's not the street angle affecting my readings.
Oil in the pan remains pure golden- never seems to turn black, in fact, reading the level is difficult because the oil is so clear. No water contamination and no fuel smell...just seems like out of the bottle lubricant- always.

Coolant level changes (overflow tank in place, return line new, new pressure cap)...sometimes it stays full, other times I have to add a pint or three. Car sometimes billows white smoke- usually at startup but then just randomly- sometimes under load, other times at cruise. From a cold start, the first time I cross 4k RPM I usually leave a 100 ft. smoke trail (white smoke) but this is not consistent either.
No bubbles in rad filler neck with engine running.

Oil temp (measured with the dubious SPI gauge) runs @200F, usually a bit lower at speed but sometimes climbing to 240F for no apparent reason.

So I'm have trouble reconciling all these conflicting events.
Things seem to point to a motor that's ready to go, yet the car drives wonderfully- if I blocked off the gauges and ignored the oil/coolant level variations, I'd think everything was just dandy.
Problem is, I want to take the car on an extended (3K miles) trip but I just don't trust it and really don't want to pop a motor in the middle of Nebraska.

Any theories that can tie all these symptoms together or any further tests I can run would be welcomed.