Sorry in advance for the long introduction, just didn't know what might be relevant to the problem and didn't want to leave anything out. Anyway, here it goes...

I got my car almost 3 years ago, and it has never had any major issues since i've owned it. However when i bought the car (from my mechanic) i was told the oil had been changed right before they sold it to me, so i set myself a reminder to do it 3,500 miles later (i know some people say 5k but figured 3.5k was fine) and didnt think much of it. So 3,500 miles later I send my car to get an oil change and when i go to pick it up and ask them how it went the guy goes "well we were going to change your oil, but there was none, nothing came out when we pulled the sump plug". So here I am thinking "oh ****, I burned thru all my oil and my engine is shot..."

but they said they cleaned out the engine (lot of sludge/burnt oil i guess) and added new oil. this was not my actual mechanic tho, just some jiffylube type place that i had a coupon for a free oil change at so they were not able to diagnose the cause. I drove off in the car and it drove just like before i brought it in; it felt smooth, pulled hard, and seemed to be in as good as new condition.

my first question is: how could i run my engine on NO oil and not destroy it or at least have some check oil light or something come on in the dash and have no noticeable decrease in car performance?

ever since that incident i have checked my oil carefully every tank (about 300-350miles) and have had to top it up just a few times. however how often i have to top it up varies greatly and can range from a quart every 2500 miles, to a quart every 500 and it seems to depend on how i drive altho not completely certain (but i think when i redline a lot it burns much quicker, makes sense i guess). there are no tell-tale signs like oil around the head or drips on the ground under the car either. car does not have a noticeable color smoke coming out the exhaust (altho it has some black soot at the tip, but dont know if thats normal, most cars seem to have it to some extent)

so my other question is: what are some possible leak locations that it could be based on the description, and how extensive/expensive would it be to fix?

oh, and I have not had my mechanic diagnose it because they said it may be really easy to do or it could be up to $500 just to DIAGNOSE if it is not one of the easy to pinpoint common causes. due to not having $500 to throw on that I have not had them do a full diagnosis.

 

The general rule on "normal" oil consumption is that if you can go more than 1000 miles per quart and there is no visible smoke, it is normal. Very hard driving could use more. It will naturally increase as the engine wears until at some point you decide that the engine is worn out, but you seem to be still well away from that point.

It is a common misconception not to do anything about oil except to change it every so often, a lot of engines get ruined that way.

 

so are you basically saying that if I have no smoke from the exhaust/engine bay then it's basically nothing and i should just top off oil as needed? what about the times i burn through a quart in 500miles (but no smoke), can that still be considered normal for a healthy engine? mine has 125K on it by the way if that changes anything.

i realize it's probably not that serious given that the car drives absolutely fine, but just wanted to make sure it wasn't burning through an unreasonable amount of oil or the start to a serious problem.

also is there any basis behind "high mileage" oil like Castrol's GTX High Mileage? it claims to be better for engines with over 75,000 miles and stop/slow small oil leaks while maintaining the same weight oil

 

Before I had my engine rebuilt, I was using a heavier weight oil. 20W-50 Castrol seems to work well for me.