Hello I will help you with your question,
The first thing you need to do is remove the oil pressure sender and install a mechanical gauge to see if the pressure is really fluctuating. If it is there may be debris on the pressure regulator seat or the spring may have broken
Let me know if it is the electrical system (mechanical gauge is normal) or if it is the lubrication system and we can work from there.
Can you check your year or post the VIN as Mazda did not make a B3000 in 1993
Let me know