I was looking at oil and I noticed Mobil 5000 was the only conventional oil that had a dual API rating for spark engines and diesel, SM/CF. And Mobil 1 and Castrol Syntec were the only synthetics that had the dual rating of SM/CF.

What is the significance of the oil having a dual rating besides use in a diesel and for gasoline engine use? I imagine some of these other oils probably are CF rated, it's just not listed on the product but is in its datasheet. Also, CF is a pretty low/old diesel rating, so what's the signifance of a higher or lower diesel rating in regard to gasoline engine use?

It means the oil is slightly more robust than an SL/SM oil with no C rating......

Yeah I figure it means the oil has a higher TBN, possibly detergent etc. I guess what I was asking about is the specifics and the pros and cons of having a Cx rating or a higher/lower C rating for use in gasoline engines without having to do a bunch of researching it myself.

Oils like Delo,Delvac, Rotella, etc., also have dual ratings .
I don't think there is significance in the dual rating other than what it plainly stated.

As to having an older diesel rating, it may have more sulfur.

Almost all oils on the market here (except Motorcraft, Mopar and ACDelco amongst a few others) have a dual gas/diesel rating - the most common being SL/CF, but there's one or two cheaper SJ/CF oils available.

With a CF rating, I do not think there's too much of a benefit, although it's probably more robust than a standalone SM oil in your case.

Now if it's an oil intended for diesel use with a gasoline rating, such as Pennzoil Long Life Gold, I reckon it's a much tougher oil - a CJ-4/SM rating in case of Pennzoil LL Gold.

I don't think it matters which rating comes first or second.
If the ratings are there at all, they apply fully.

Well let me ask the question a different way. What is the difference between a CF oil and a higher rated on like CJ?

Hi,
mechanicx - You asked this:
"Well let me ask the question a different way. What is the difference between a CF oil and a higher rated on like CJ?"

Answer - a great deal!

If you use a CF lubricant in an engine calling for a CJ-4 you will have problems - usually major ones!
Mostly these will be typically but not exclusively deposit related. CJ-4 lubricants have much better soot handling abilities than CF versions amongst many other things

If you use a CJ-4 lubricant in an engine that specifically calls for a CF lubricant and the same situation is likely to occur. Mostly this will typically be glazing and etc

Many CJ-4 lubricants are rearwards compatible - some are not!

The CF spec. (from around mid 1990s) requirement was usually directed at pre-combustion type engines and where higher sulphur fuels may be encountered. It was not directed at heavy high speed heavy diesel engines of that area - CF-4 was!.
Now, high speed late technology small displacement diesels also easily use CF

Mechtec2 - The order of the API's ratings denotes the "primary" purpose of the lubricant. If the "C?" rating comes first it is a HDEO as we call them. If the "S?" rating is first it is primarily a PCMO. The formulations apply accordingly

Thanks Doug Hilliary. I guess what I'm getting at is could an SM oil that also has at least a CF rating be better and a better oil for a spark engine than one that has no diesel rating? And could an oil that has a SM rating and an CJ rating be even better still than the other 2 examples for a spark ignition gas engine?

Hi,
mechanicx - You said:
"I guess what I'm getting at is could an SM oil that also has at least a CF rating be better and a better oil for a spark engine than one that has no diesel rating? And could an oil that has a SM rating and an CJ rating be even better still than the other 2 examples for a spark ignition gas engine?"

1 - The CF rating on an SM lubricant is only relevant if it is to be used in a corresponding diesel engine. That said it has been commonplace among many engine makers to require both (ACEA's A3/B3, A3/B4, A3/B3/B4 for example) for some time (Porsche since around 1975). It is perhaps a bit of a carry over and most PCMOs today would almost fly through CF compliance. typically it was S?/CD - we have moved on!
It also saves on inventory and reduces the risk of "errors" at car dealerships!

2 - Very few SM lubricants have a CJ-4 rating. The formulating and testing for CJ-4 compliance (a "Commercial" lubricant) is costly and typically is not needed as they (SM) are consumer and not Commercial lubricants

OK I guess that answers my question. The diesel rating an SM oil that's used in a spark engine has or doesn't have is irrelevent. Like you said, in the past I noticed all oils seemed to have a dual rating on the container, but now not so much.