The API Engine Oil Classification chart for 2002 show both 15w and 20w classes as ranging upwards of 5.6 viscosity (cSt at 100C). The next range starts at 9.3 for 25w and 30 weight. So it seems that both 15w and 20w range from 5.6 to 9.3. Is there a split in that range below which is 15w and above which is 20w? Or is the difference in HT/HS viscosity, but the chart shows 15w, 20w, and even 25w all having high-sheer-rate viscosity (cP at 150C min) of 3.7. Am I confused or is it really pointless to get a 20w oil because the 15w will be just as good?