99 S-10 2.2 oil pressure fluctuations

An engine creates oil pressure as oil flows through restrictive passages inside the motor. An oil pump DOES NOT create pressure, it only moves oil a certain volume of oil from one place to another. Resistance creates pressure. There is a plunger valve that is controlled by a spring somewhere in your oiling system that releives pressure or (resistance) in your engine. A rule of thumb is every 1000 rpms requires 10psi of oil pressure. Why did I tell you all this---Well it's important to remember that high oil pressure is just as bad or worse than low pressure.  In your case, the pressure relief valve spring may a worn out or broken. Every engine is a little different with the variances in oil pressure. It depends on the restrictions. If it's easy to check, then check the oil pressure releif spring for any weakness, however 20psi at 2000 rpm is just about perfect.