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How to Read Your Oil Can

You can choose the correct API rating for your car


Corvette Motor Oil Filter

Always change your filter when you change your oil - and make sure your oil is right for your 'Vette. You can read the API standard for the oil on the bottle. For older Corvettes, make sure you get the older SL standard, not the newer SM standard.

Photo courtesy of Royal Purple

A while back I posted a quick tip on how to select a good engine oil for your Corvette, based on the amount of zinc phosphate (ZDDP) added to the oil mixture. This zinc was used to protect cams and lifters in older (pre-1989) engines, but has been phased out of modern oils because it destroys catalytic converters.

I'll reiterate the bottom line - if your Corvette is a 1989 or later model, use the grade and viscosity of oil specified in your owner's manual. That's easy. But if your Corvette is older than 1989, you have a little research to do. Fortunately, I did it for you.

I had a chat with the oil engineers at Royal Purple - they make synthetic oils and cater to the racing community. Obviously, they know their stuff. The question I asked was this - "How do we know what the ZDDP content is in an oil we buy over the counter? It's not listed on the bottle." Here's what they had to say:

All API-certified oils contain an anti-wear additive, like ZDDP (Zinc Dialkyl DithioPhosphate). However, current API ratings have reduced the maximum amount of phosphorus, and therefore ZDDP, allowed in a motor oil. The current API SM rating is the most restrictive. These API SM oils are generally not suitable for flat tappet camshafts due to the reduced amount of ZDDP. Royal Purple has retained the older API SL rating because of the required reduction in anti-wear.

What this means is that you can check the API rating (which is in a little starburst logo on the bottle and on the case) and know about how much ZDDP is in the oil you're buying. SM-grade oil will have less than 600PPM (parts per million)of zinc, while SL-grade oil will have about 1100PPM of zinc. Higher viscosity SL oils (10W40, 15W40, 20W50) have a higher level of zinc at about 1200-1300PPM.

So if the engine in your Corvette is stock or very near to it, any SL-rated oil (for example, SAE 10W30) is a good choice. For a little extra margin of protection and/or if the engine has a more aggressive flat tappet camshaft installed, an oil like an SAE 10W40 would be a better choice. Note that there are many SL-grade oils on the market, and you should read the bottle and download the official API engine oil guide to help you select your oil.

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