"if the oil sending unit is bad will the mechanical guage save me from replacing the sending unit"

In a word...NO.
And it sounds like the sending switch is faulty. It may be clogged; or there may be crud in the gallery it is connected to.
The sending switch is buried back by the distributor and has a wire connector coming out of it. (not to be confused with the crank sensor which is also located way back there but on the trans. bell housing) After you find it and remove it, pack some rags around the area and have bud crank the engine over while you watch the area. CAREFULLY!!! Spinning engine parts and spraying oil are hazardous for obvious reasons! If a good gush comes out, the gallery is clear and the switch is suspect. If no oil; further flushing is needed...or worse. But we won't go there just yet.

Good Luck!

 

thnx do you i can still drive in the mean time right? because i gotta drive from melbourne/palmbay to tampa this weekend

 

I can only tell you what I would do, and that is test the oil flow 1st. When you are sure its only the gauge, driving the truck is no problem although you may get a gauges/mil dash light.
Still another thought on this is that there is a plug in that same gallery between the sending unit and the oil filter ports. This plug essentially keeps the oil flowing in one direction through the filter. While not common, that plug may have worked it's way loose and further down into the gallery or may be missing and went down to the oil pan. If missing, it could make the gauge give the erratic readings you've been experiencing. Like I said not that common. When you have the sender out take a stiff wire and fish it down the opening. If it stops (about 7-8 inches down the plug is still there. If it keeps going...uh-oh.
Again, check you flow BEFORE you hit the road. I don't want to have to drive to Tampa to rescue you. I hate Tamper!
WC