Rode for 20 min. Engine light & oil light came on. Oil level perfect. Kept running with limited power (5mph) Loud continuous beep. Now runs fine. What should I have service check?

If you cannot diagnose/fix the problem yourself, it's time to take the vehicle to a qualified, certified, automobile mechanic.

Check your oil level and antifreeze level or change your oil if the problem continues

check oil and coolant levels . Run the fault codes to check what put the engine light on.

Sounds like the Needle valve in the carburetor is stuck 'down' letting gas continually run from gas tank into carb. When the valve can't go up (Close) gas will run past the carburetor and into the crank case I bet your oil is full of gas. Check oil level see if it is overfull

that is a sign of the wrong oil grade. that truck takes 5 w 20 oil

replace oil level sensor

Did you check your oil level? Did you check your oil pressure? Or do you plan on ignoring the warnings until your engine blows up and will no longer run?

If it is 2-stroke engine check oil level and the oil level sensor.

Sounds like a low oil sensor. Check engine oil level. If oil is at the corect level and problem continues replace sensor.

Unfortunately, you've got serious problems. The GM 3.1 and 3.4 engines have a nasty habit of breaking camshafts. For some reason, which I have yet to determine, the cam bearings seize on the cam and the camshaft ends up breaking in half. The engine will continue to run, albeit very poorly since all the cylinders are not working because the rear half of the camshaft, which runs the oil pump, is not spinning.