What kind of oil should i use ?

Got 96 avenger with a v4 2.0 liter motor.. what kind of oil should i be using ? 5w-30 ? 10w-30 ? it doesnt say it on the oil cap.

I would use 10W-30 or 10W-40 because it's an older car. 30 weight for the summer, 40 weight for the winter.

ever since i put in this 5-30 synthetic oil the rpm is a little unstable.. but its fine once i take off.. So 10-40 is thicker oil ? it only has 83k miles on it

5W-30 is like water for your engine w/ 83k. Use 10W-30 or 40 for best results and to prevent possible engine problems.

I don't have the manual for the car but if I remember right, it's 10W-30. Just because an oil weight is thin doesn't mean that it's no good. Your engine requirements for oil weight are based off of what the engine needs to properly function. The oil weight determines how long the oil will stay on the walls of the chamber before falling back to the bottom. If the oil falls to quickly for the piston to use it, then the piston will scratch the cylinder walls and cause piston walk which will eventually cause you to blow your motor. On the other hand if the oil weight is to heavy, it will cause the oil to stay on the walls longer than needed which can damage your piston rings. It does this by causing the piston rings to push the oil off of the walls in more amounts than what the rings were designed to do. This will also lead to compression problems an you may even create a leak throught the rings, burning oil in the combustion chamber.