Mechanics

DOES OLD OIL HARM A RELATIVELY NEW ENGINE, TO BE ADDED?

1998 Toyota Corolla • 2.0L 4 cylinder FWD Automatic • 30,000 miles

I just have a simple question: . Same type, brand, etc, oil. Does old oil stored outdoors in a car trunk go bad and cause any harm to a relatively low-mileage engine? It's the same type, brand, etc, oil? How long does it take to get an easy question answered by you guys? 24 hours? Several days?
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ChrisStevens
June 16, 2014.




Oil doesn't go bad from age but really old oil may not meet the needs of your engine. Oil is oil and will always be oil. It's the additives in it that are of importance. There's corrosion inhibitors, detergents, friction modifiers, and viscosity index improvers. Those are the things that wear out, typically in about 3,000 miles. Normal engine heat helps deplete those additives.

Almost any engine oil you buy today will meet your engine's requirements, as long as you use the right viscosity. In your owner's manual it will specify a rating like "SG", "SF", or something like that. You'll find that rating on the container. The "S" stands for "spark ignition", in other words, a gas engine. Many oils will also have a "CD" or "CG" rating. The "C" stands for "compression ignition", meaning a diesel engine. Every time there is a significant improvement or development in the oil, it will get the next higher second letter. An oil rated "SF" is one step better, or newer, than one rated "SE". I don't even know what we're up to now, but we were up to "SG" quite a while ago. I'm thinking that was the rating about ten years ago.

New cars usually call for the latest rating, but when you have an older engine that called for an older rating, you can use oil with that or any higher rating. That means if you need an "SG" rated oil, you can use "SH", or "SI" too. You should not use "SE" or "SF". If you do use the older-rated oil, there's a good chance no engine damage will occur right now. It's more likely that if you were to develop an oil-related problem, the wrong grade will make that occur sooner, that's all.

We typically don't know what specifications have to be met or improved to warrant the newer, higher rating. If it's better anti-scuff properties, and your engine design needs that, the bearings could wear out faster. Rather than worry about things like that, all shops use the latest oils. They don't store large quantities that could take years to use up. When a manufacturer develops a new engine design that requires an improved oil, it will either be on the market already or they'll make sure owners know if there's a specific oil that must be used.
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Caradiodoc
Jun 16, 2014.
Thanks greatly for the detailed answer, it's way more professional and informational than I hoped for. Kudos. I'm temporarily laid off, and your recent home fire makes me realize I have nothing to complain about. I am truly sorry as a Christian hard-working kid (female) that you're going through that right now. Still, the future is bright for the smart, honest, ambitious & caring people of this world, I'm convinced of it.

Tiny
ChrisStevens
Jun 17, 2014.
Nice work Caradiodoc : -)

2CarPros Ken
Nov 11, 2014.

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