Just for you, H.J., because you are being a bud, even though you are buggin'. I apologize, it's been a rough start to this week.
The really best oil is a single-weight (like 30). The fact is that multi viscosity oil sucks because it has to have "viscosity-improver" additives in it to create the viscosity changes from cold to hot. But the reality is, that most places where you live, there are significant enough temperature variations to require the use of a milti-viscosity oil. My manual suggests that I idealy use a 10W40. Remember that the "W" stands for "winter" not weight. I would suggest that grade for you in northern Florida as well. Or switch to Mobil 1 and run the 15W50...
On the other hand, the American sales rep for Liqui-Moly (Lubro-Moly in the USA), claims that he uses their Völ Synthese 0W40 in his Suburban, and he lives in San Diego. So why does he want to use a sub-zero, polar climate oil with a 0W rating? He told me he wants to get maximum immediate oil flow to his bearings and journals at start-up.
Running the thinnest weight you can that maintains adequate oil pressure at idle sounds reasonable, but what happens when that oil reaches high temp in the summer in stop and go traffic? Will it adequately protect your crucial engine mating surfaces? Especially on a higher mileage engine? Another rule of thumb is to run a milti-viscosity oil that is as close to the two opposed numbers as possible. Say, 10W30, or 20W50 to minimize the amount of "viscosity improvers" in it. Also, some people prefer to run a thinner oil in a colder climate, but a thicker one in the summer.
See, it isn't cut and dried at all. It's very subjective, and especially subject to people's personal beliefs in what will be best for their car.
As for the rating system for motor oils, there are only two categories: S is for service station grade. This is for most passenger cars. Including diesels. And C is for commercial service that includes taxicabs, trucks, farm tractors and the like. It really doesn't have so much to do with spark-ignited and compression combustion engine differences as it does with the type of demands you put on the vehicle. As for the second alpha character, the further down the alphabet it is, the better suited it is for the changes in machining and emissions that has evolved in modern engines. That's right, not necessarily better, but certainly newer.
As these new ratings have ascended, they have done so to cope with the tighter tolerances of computer controlled milling machines, and hotter running engines with more sophisticated emissions controls. The new rating for commercial grade engine oil is CI-4. That rating is primarily for the latest diesel engines being put into trucks and buses, because that is what the commercial fleets are using more and more, diesel. An older design of a diesel can still use a CF rated oil with confidence.
The rating that Castrol had for it's oil 10 years ago is certainly different on the surface than the rating today. Because it was designed to run in those, and even older engines at that time. The oil was not "improved" to meet the new ratings, the rating sytem simply was changed. Castrol GTX used to be SF/CE, and now it's SJ, SH, and SL with no C rating (?). But is is, as a SL rated oil, in reality the equivalent of the CH-4 rating used for commercial applications. Castrol has actually manipulated these ratings in order to allow them to target market Castrol RX, which they describe as being designed for "diesel light trucks and passenger cars where API CH-4, CG-4, CF-4, CF or SJ are recommended". Note that the SJ rating is used by Castrol as an equivalent to CH-4. And check the API chart in the first thread I posted to see grade equivalents and when they were introduced. Look at the dates. http://www.api.org/programs_services/quality/oilchart.htm
You can also go to www.castrolusa.com
for the details on their products and those ratings.
My car used Castrol 20W50 during it's early pre-100K mile period. It would still be a viable oil to use in it today. I just choose to use Mobil 1 15W50 because it is a true synthetic (tri-synthetic) with a CF rating, and moerover, because my car was changed over to that oil at about 110K miles, so I'll stick with it.
I really wanted to avoid going into such detail by just posting the threads. It's all been said there before...