So my 98 Buick has an oil leak, about 1-2 qts over 5000 miles. I have a bad feelIng it’s the rear main, but that’s another story. I do 5000 mile oil changes on both my and my wifes cars. So is it a bad idea to use old oil to top off my oil ?? I’m thinking it’s fine, but figured I would light the fires of this forum 
2 quarts in 5000 files isn’t a rate worth loosing sleep over. If it is the rear seal, you should see some dripping on the pavement where you park, see any? Me, I wouldn’t ever put old oil into an engine. What does a quart of oil cost? $4 or $5? That’s cheap maintenance insurance.
+2
Yeah, just add it. Don’t worry about it as long as the adding oil isn’t a substitute for an oil change.
For two quarts at less than $3 a quart? I don’t even use old oil to oil the oil filter gasket. I wouldn’t do it. You might want to have a long look at the oil pan gasket though as the culprit. Seems to be a common issue with the Buicks. Mine was leaking a quart a week and fixed it for $20. Also like I said before, I think 5000 is a bit too long for that vintage. More like 3-4000 would be more like it.
Nope, don’t do it. Your car still has lots of miles on it if you treat it right. New oil only.
I do it, but I wouldn’t do it in your case. I use synthetic oil in my newer vehicles and change it before it is “worn out”. It still has some service life in it. when I have a vehicle that is consuming oil at 500 miles/qt, then I start using the old synthetic oil as makeup oil. I still do regular oil changes on the vehicle in question with fresh regular oil.
Oh, just noticed the adding ‘old oil’. Was wondering why everyone was disagreeing but now that makes sense.
Don’t add old oil. Oil is cheap.There’s no point.
I also thought pan gasket, and I resealed it a few months ago. No luck still leaking, and yes I have a mark on my driveway and it’s driving me nuts. The bolts on the oil pan are dripping oil ( small drops). The Motor seems dry above the pan, so I thought for sure it was the pan. The only thing I can think of is the rear main and the fly wheel is fanning it back onto the pan (being fwd I don’t see any other way). As a PS I have been doing 5000 mile oil changes for over two years and the pan was clean as a whistle when i pulled it.
Check that oil pressure sending unit.
When you say “old oil” do you mean using a case of oil that you found that has been sitting in the back of the garage for 8 years or do you mean used oil that you have drained from a previous oil change?
Old oil should be fine to use. Used oil? Well, I don’t even know how you’d get it back into the car after draining it. Wouldn’t you have to pour it from the drain pan back into the bottles? That process alone isn’t worth the price of new oil, in my opinion anyway.
With tongue in cheek, I might say that since many car makers recommend 10k miles oil changes and you do yours at 5k miles then the old oil should still be just fine…
Ditto on the oil pressure sending unit as that is not that rare a problem and it does provide a way out for oil feed pressure.
When I was in high school, I had a old Corvair that leaked oil like crazy - a quart every few hundred miles. I worked in a gas station and often took the oil we drained from customers’ cars and used it in my Corvair. When a car is on its last legs and you don’t have any money, it sure makes you look at the problem differently.
With the OP’s car burning a quart every 2500 miles, I’d only use new (ie non-previously-used) oil.
rather than used drain oil, I’d buy the cheapest new oil I could find - ie Walmart brand oil.
I agree with Uncle…as usual. Putting back old oil kind of defeats the idea of doing an oil change. Not that there will be a measurable effect, but using fresh, cheap but API certified oil would seem reasonable to me. In the old days, I couldn’t sleep at night thinking that a garage did this to my car. Of course as I get older, I have trouble with that regardless.
OP, you must be more festidious about the pans and storage containers you use for drainage. I never remember what I last used them for…could be a little antifreeze, transmission oil, hydraulic fluid with metal particulates etc. in there. I don’t use separate pans for differentials, outboards, snow blowers and tractors, many of which use unfiltered oil and fluids of varying types. You’re braver then I.
My car leaks/burns a quart every 2 months. I use the OLM and it says I can go 7k miles which is usually about 10months. So when my oil change is due, I have added almost 5 qts of fresh oil. Should I ever change my oil? Or maybe just the filter?
Using old oil willl certainly speed up the demise of your car. I had a Chevy when in college, and it used a lot of oil. I bought the cheapest oil (30 cents a quart at that time) by the case and just kept topping up. My one difficulty arose when crossing the US-Canada border with a fresh case in the trunk, and having to explain I was not smuggling oil, but needed it to keep going!
I wouldn’t do it. Over time, you will end up topping off with oil that has 20,000 miles and more on it. Not the entire replacement charge, but a part of it will be quite old. Buy Wallyworld oil in the 5 gallon container.
I’m with asemaster. What do you mean by “old oil”?
First thanks for all the replys. So let me clarify, I have been driving 8-10,000 miles a year. So this was moot point. However I just took a new job which I suspect will be a 30,000 mile a year job from what I am being told. I need this car to last 1-2 years while I save for another car. Now that I am driving the car more the leak maybe getting worse, although it’s too soon to tell. As for old oil, I did mean oil that has been used in another car. Specifically my wifes mini van as we use semi sinthetic in it and I still drop it at 5k.
If you hope to get the maximum value from that or any other vehicle keep the oil changed according to the owners manual and never use “waste” oil. Ever hear the expression “Penny wise and Pound foolish?”