1996 Chevy Cavalier Noisy Engine - grind sound.

I had a really loud, almost grinding sound coming from engine area. The car was due for an oil change so I had it done about 2 weeks ago and the sound was less prevalent. Now it is back.

I have a certified mechanic friend who, 2 wks ago, said the oil might not be lubricating the engine as good as it used to, causing the slight shake and noises. He said I could add something to the car that would help with lubrication (but never found out what it was he was talking about).

I know the O2 sensor is bad, and was wondering what that will do over time? Could that add to the problem? Really any thoughts would be appreciated.