Mostly because of DI and the soot particles it creates together with low friction/tension piston rigns that aid fuel consumption and emissions. Not sure if you've observed but DI engines change the color of the oil a lot. Migth be also be due to increased compression in todays engines but most likely a combined factor. We also have balance shafts, more beefy cooling that requires to be driven, more stress on the chain with thinner oil and lower part tolerances. I'd need about 12 beers to go trough all of it.

See your vehicles service manual for change intervals. If your vehicle is doing short trips (e.g. city driving) or used extensively under heavy load (e.g. towing) then change more frequently. Generally the vehicles that recommend API SP oil have engines optimized fir efficiency rather than longevity so more frequent changes are recommended to assist the engine with a longer service life.

Yes, sp is backwards compatible with earlier specifications so long as it's the same weight. On the flip side if your car requires sp or later, you'd have to use sp as it's the latest since 2023.

@@aslkdfjhg Thank you

Later specs always cover the previous ones.