10W-40 Motor oil is not as popular anymore. So what the oil refiner did, to make money, is sell it to Wallyworld for the older cars that run on SL oil, at the price Wally wanted...lol
5W-20/5W-30 are the most popular fills for the newer cars to meet EPA/API/ILSAC Standards/10W-30 is probably less popular, but still produced.
10W-40 is the least popular nowadays, except for people who love the thicker oil because "thicker" is better, or the car manufacturer wants that type oil put in the engine. I am not trying to start thick vs thin oil debates between everyone....please....
SL is still good for the manufacturer's cars that use that rating. Still plenty of those cars around nowadays.