CI-4 oils (mineral oil or synthetic based) are formulated from similar base stocks as SL oils, but have a higher concentration of critical detergent, dispersant, anti-wear, and anti-corrosion additives. The level of these additives in spark ignition oils has actually been reduced over the years because their combustion byproducts are not catalytic converter friendly and also because modern engines that feature more precise fuel control and quick warm up, don't need as much of these additives.
I use CI-4 oil in '63 Corvette and '76 Cosworth Vega both of which I will probably keep forever, but continue to use SL type oils in my two "modern" cars including my '88 201/2.6. Since these cars don't typically see temps below about 45F I use 20W-50 - any major brand mineral-based oil that is on sale. Since I only drive them about 1500 miles per year and change the oil once per year, I don't need the long life properties (more resistance to oxidation) of synthetic based oils.
The most common multivis grade for CI-4 oils is 15W-40, so that's what I use in the old Chevies.
See Patrick Bedard's column in the November '99 Car and Driver for a more thorough discussion on this subject.