GF-4 of course is the current spec - the drivers came from the automotive industry, mostly due to extended warranty requirements placed on them by states like California, where the emissions system has to be functional out to 100,000 miles. Studies indicated that the phosphorous, primarily from the antiwear addtive ZDDP, could "foul" the catalytic converters over time. So the mfrs. charged the oil industry to develop lower phosphorous oils - since the zinc which makes up the other component of ZDDP must be balanced with the phosphorous, its level had to be dropped as well. The challenge to the oil industry was to develop additives that offset the loss of protection that resulted from the lowered levels of ZDDP. Working with the major additive suppliers like Lubrizol, the major oil companies have reformulated base stocks, often using 2 types, as well as special fluids called "correction fluids". Ideally, a single basestock with no correction fluid would be preferred, as it lowers costs, and reduces the chance of error in blending. They have also gone to costly amine compounds, and increased the molybdenum disulphides, to keep wear protection high. The result is that today's better minoils are even closer to the protection and longevity that was once reserved for synthetics. The gap is all but closed, except perhaps at the extremes of cold and heat, and in the service life.

Already, continued advancements in refining processes are producing Group II+ and Group III+ fluids - narrowing the gap even more.

BTW, there are actually now Group VI fluids - called PIO's, they were developed in Italy, and are not available in the USA yet - we will see what they bring to the table in the years ahead.

Here is some good info:

http://www.chevron.com/prodserv/BaseOils/gf4_faq.shtml