Although mono-grade motor oils (i.e. SAE 30, 40), also known as straight grades, represent less than 5% of the motor oil consumed in the US, PQIA receives a notable number of inquiries about them. For this reason, we decided to take a look these grades.

Whereas most of the mono-grade motor oils examined by PQIA meet the specifications they claim, it is important to know that some of the specifications stated on mono-grade labels are obsolete and use of such oil can cause harm to modern engines. Further, although some labels on mono-grades include language to help guide consumers to make the appropriate buying decision, others lack such language.  

Click bottles below for details on the first brands of mono-grades examined by PQIA

PQIA receives a notable number of inquiries about mono-grades
 

Most of the questions PQIA hears about mono-grades come from consumers asking if these motor oils are quality products that can be used in their cars. Since the mono-grades on the shelves at convenience and other retail stores are typically offered at a lower price than multi-grades, why shouldn't consumers save some money buying a mono-grade?

Whereas the default answer to the questions is; Refer to your Owner's Manual or consult your car manufacturer and/or Service Dealer, most in our industry know what they will find when they go there.  It will not be a recommendation for mono-grade considering nearly all passenger car OEMs specify the use of multi-viscosity motor oils for cars currently on the road.

Although most of the cars currently on the road do not specify the use mono-grade oil, there are still some vintage and classic cars out there that can still use it. Most commonly these include vehicles built prior to 1950 and those operating in warmer climates, pulling trailers or other heavy loads. Mono-grades may also be appropriate for use in some manual shift gear boxes in older cars, diesel engines, older motorcycle engines, such small engines as lawnmowers, and in some compressors, hydraulic systems, and other niche applications.

But whether the owner's manual calls for a multi-, or mono-grade motor oil, most owners' manuals also specify America Petroleum Institute (API) Service Classifications, and/or other industry and OEM specifications. This takes us to another critical issue to determine if a mono-grade is suitable for use in your car.

Even if a mono-grade is acceptable for use in your vehicle, it is very important to check the label on the oil to assure the performance level(s) shown on the label match up with those specified in the owner's manual of your car. To illustrate why, consider that many of the mono-grade motor oils PQIA observes on store shelves meet either an API SA, or SB Service Classification. Oils meeting an SA classification have no additives and those meeting an SB classification have very little additives to protect your car's engine, and they are formulated for vehicles built prior to 1930, and 1951, respectively. That's a long time ago and motor oils have certainly changed a great deal since then. In fact, today's motor oils are backward compatible. This means motor oils meeting the current API SN specification can be used in vehicles built in the 1930s on up.

So don't be fooled, whereas mono-grades have a home in some applications, most are not appropriate for use in motor vehicles currently on the road.