Oil 2.25 quarts low / engine damage

I need your advice.

I checked the oil something lik 7,000 miles after my last oil change and....2.25 quarts low. (that's how much i added to get full).
Maybe even 2.5 low.
Ow..

For reasons, I did not change oil then. Of course I regret that.
Then a few thousand miles later, I had to add a 1/2 quart.
I changed the oil at like 10,400 (I know).

Half my mileage is 12 mile trips. The rest is mostly 1-2 mile trips.

So, here's where it gets unfortunate, and I need advice.

Currently, my engine makes a vibrating purring sound, that irritates me when I'm in the driver's seat and car is at a stand still. Like drdrdrdrdrdrdr, but at a faster rate that one would make if you made that sound with your mouth. . I guess I'd say 'quieter' than a voice. I guess.

My mechanic says that basically the oil is not getting to the pistons in time.

He wants me to do this:
As soon as almost a quart leaves the engine (as we know it will), he wants to add an oil additive that he says "should help", though there will be no way of knowing if it does.

He also says the purring may or may not stop, but either way, there will be no way to tell if I've still got a problem. At the end of our conversation he stated that it should be ok.

He definitely wants to do what he can or "risk ruining the engine".
I hope I haven't already.

So, what's exactly going on, and what should I do?

What's the risk....should I even keep this car?

I am required be law to tell any potential buyer of any problem I know about the car.
Yet and still, if the risk of dead car or short life / expensive repairs is significant, I'd rather sell it to someone willing to take on that risk than deal with major problems myself, of course.

But if it 'should be fine', fine.

Cliff notes:
Engine oil down 2.5 quarts
Now purring/constant 'rough hum'-type vibration/sound
Damage assessment ?
Next steps?
In the future: Risk of major repairs?

Thanks,
SL

 

@ETC(SS)
In another thread, you said, "...a trusted local mechanic then you can certainly clean the EGR and the throttle body, and replace the PCV. Those are easy, sub $200 items."

>> Value in that / reason? I just want to see how important this is before spending $350. ( only because you said "you can certainly...", which sometimes means -- no one knows if this helps? Hard to tell from your wording.

"DO NOT go to a Toyota dealership and mention throttle body cleaning otherwise you'll wind up paying $150 for a $4 can of throttle body cleaner."

>> Got it, thanks.

"Clubbing up to a 5w30 flavor of oil won't really hurt either."

>>OK, will do.

"The problem you're going to have now isn't the rings in your engine, but rather your catalytic converter.
You see.....in 45-1/2 states, a bad catalytic converter is about a $300 fix.
It's a little different in SoCal, and you're going to have to get a handle on the drinking problem before your cat gets fodded out and have to deal with that as a main issue."

>>I'm in KY / Ohio. How will I know when I need that fixed ?

Thank you for advice!!