Oil is leaking somewhere under the engine

I have a 2005 Nissan Altima SE V6. The car has 163,000 and I have changed oil services ever 4-6 K miles. I take care of the car. I changed the spark plugs June of last year in the 140K mile interval. I took longer than expected. On that repair the car was showing an oil leak.

The major repairs this car has expierenced has been engine mounts (3); some air conditioner repairs; normal brake pads change, power steering pump change; given monthly gas additives; put regular 87 gas recentley first 5 years 93 oct; spark plugs 2011; transmission change oil 2 times; had a crash on the back no damage to mechanics.

1- The dealer charged me over $ 500 to fix this oil leak. How can oil leaks be fixed?

2- on an oil change at jiffy lube the failed to put an air filter so I drove the car almost 4 months withot an air filter. Apparently this caused recently to have the car change a
air intake system $ 550 dollars. This part on the internet costs $ 250.

3- The car is showing the light "check engine". Sometimes it comes out from me not
fasting enough the gas pump cover. But this time the car is showing an acceleration
idle that is hard and fast.

What do you suggest is showing this check engine light status? The drawback is a rough idle and hard pedal. Recently my car stalled on the highway shutting all systems down and the dealer changed the air intake system stating it was too dirty (could have been from the time that the air filter was out).

This car has a history of problems with the chain valves.

What are the chances it is a valve issue, an oxigen sensor issue?

Can you drive the car with this check engine forever? If it is an oxigen sensor issue?

Thanks! Jorge Lopez