Why does my 1999 manual transmission chevy S-10 oil gauge go up and down as I drive?

indicates a problem oil pressure sender unit
have the oil pressure checked using a mechanical pressure gauge and if OK then replace the sender unit or gauge

but where is the fuel pump oil pressure switch located ? my 98 has an oil pressure sending unit with single wire for oil pressure gauge I believe near the filter on passenger side, but I cannot find the fuel pump oil pressure kill switch ?

1999 cavelier manual trans: My son was shifting his manual trans really rough (ramming through the gears to try to beat someone out at a light) and ended up not being able to put it into anything but 3 and 4 (straight up and down). We noticed that the shifter cable, that actuates the position of the arm on the transmission, had pulled away from a "c" clip and was not held into place about 8-10 inches down the cable away from the trans. The cable needs to be secured into that clip in order to let the wire inside move the trans. lever to shift. Otherwise you will be stuck on what ever "rail" your trans is on at the time of disconnection. It would be the first thing that I would check.

Check ALT fuse .
Is it still charging?

LEVEL CHECK With Oil Level Gauge Fig. 1: Manual transaxle oil level check - with level gauge

1. Drive the vehicle to bring the transmission up to operating temperature.
2. With the engine stopped, remove the oil level gauge front the side case of the transaxle.
3. Check the color and smell of the oil. If it is black in color or smells burnt, further transmission service is needed.
4. Wipe off oil level gauge with clean cloth.
5. Fit the oil level gauge to the transaxle side case so that the threads rest on top of the case.
6. Remove the gauge and check oil level. The level should be between FULL and LOW level line.
7. If the level is below LOW mark, add oil until the proper level is reached.

With Oil Level/Filler Plug

1. Drive the vehicle to bring the transmission up to operating temperature.
2. Raise and support the vehicle safely.
3. Remove the transaxle oil level/filler plug.
4. Check the color and smell of the oil. If it is black in color or smells burnt, further transmission service is needed.
5. Transaxle oil level should be even with the bottom of the plug hole. If oil level is low, adjust oil level accordingly.
6. Install transaxle oil level/filler plug and tighten to 40 ft. lbs. (54 Nm).
7. Lower the vehicle.

First we would need to know when this occurs. Asuming the oil level is at the full mark!. If the pressure goes up during acceleration and down at idle when engine is fully warmed up, then this is normal. An engine requires 10 psi of oil pressure per 1,000 rpm ( ex. 20 psi at 2,000 rpm, 25 psi at 2,500 rpm, and so on ) . If the gauge is moving back and forth erratically at idle , i would say the gauge is bad. If it drops to 0 while accelerating I would say your oil pump is bad. If from time to time it seems to work and other times it seems not to work, I would say the oil pressure sending unit for the gauge is bad. If the pump failed you would start to here a Knocking noise from the engine in a short period of time. Use the above scenario's to narrow down what it may be.

NORMAL JUMP AS THE ENGINE RPM INCREASES THE OIL PRESSURE GOES UP VERY FAST. I HAVE THE VAN, MY DOES THE SAME THING, U CAN TRY REPLACING THE OIL PRESSURE SENDER, BUT ONLY USE THE GM ORIG PART, AFTERMARKET ONES MAKE THE FLUCUATION WORSE..

ATF

The stepping motors in the gauges go bad. Do a search on ebay for CHEVY SILVERADO INSTRUMENT CLUSTER for cheaper repair options, dealer costs $300.00

oil pump might be going bad