Info from Shell Oil site:
The API CG-4 oil category went into effect on January 1, 1995, supplementing the CF-4 standard. Although CG-4 was designed specifically for 1994 and newer engines, it is compatible with older engines. So if you’re "topping off," you can add a CG-4 oil to a CF-4 oil without any adverse effects.
If you have a supply of CF-4 oil, you can continue to use that oil in rigs that don’t require the new category. CG-4 oils can be identified inside the API donut on the labels. These oils will be labeled as both CF-4 and CG-4. The CF-4 standard still applies to older on-highway and off-highway vehicles. If you own a 1994 engine, you may want to double check the manufacturer’s recommendation to see if only API CG-4 oils are suitable for use.
The CG-4 category oils lubricate the newer engines designed to meet federal emissions standards. These engines have been modified in ways that force soot which used to go out the exhaust into the oil. Oils with CG-4 classifications have passed tests that measure the ability to control deposit formation and inhibit oil thickening due to soot. Shell Rotella T® met these requirements in early 1994. It also meets both CF-4 and CG-4 performance categories.
At extremely low temperatures (under minus 20 degrees Fahrenheit), a synthetic motor oil can give you better cranking and pumping characteristics. You may also get better oxidation stability at extremely high temperatures--in the range of 700 to 800 degrees Fahrenheit. But the highest temperatures of oil-lubricated surfaces inside a properly functioning diesel engine won’t exceed 550 degrees!
A synthetic oil’s extreme-temperature performance is due to its man-made consistency: Every molecule looks the same. But this chemical property also means that the synthetic oil doesn’t do as good a job of "solubilizing" (holding stuff in solution) as a mineral oil. Additives aren’t solubilized as well, nor are combustion byproducts.
If additives aren’t kept in suspension, they can’t do their job. And if combustion byproducts aren’t kept suspended in the oil, they wind up as engine deposits. With some synthetics--especially in diesel engine applications--we’ve seen combustion byproducts that "plate out" or accumulate as deposits in the hotter areas of the engine: the piston crown lands and top grooves.
Deposit formations like this can cause loss of oil consumption control, and lead to more frequent engine overhauls. So certain formulation adjustments must be made to the synthetic oil to make it perform correctly with regard to additives and byproducts. Not an easy thing to do.
Then there’s the matter of cost. For the manufacturer, the raw material cost of synthetic oil can be up to four times what it is for mineral oil. (The production process costs a lot more, and so do the starting materials.) You have to ask yourself: What am I getting for my money?
Unless you’re spending most of your time in the nastier climates of places like Canada or Alaska, the diesel-engine performance of a top-flight mineral oil can easily exceed that of a synthetic. And cost you a lot less.