I work for a chemical company and in some cases you are right. with Sams you get whatever was the cheapest they can buy this week. If you are buying a synthetic you are safe with just about all the majors. Blends are okay too. the only thing you have to watch out for is some of the oils will create a sludge from parafin in the oil. I don't want to knock anyones oil. Our guys tend to use, Castrol, Valvoline, Mobil One and kendall. I have one engine in a 1972 Chevy that has over 500,000 miles. I have a 1990 Chevy truck with a 350 in inthat has over 550,000 miles.
the main thing you have to do with oil is change it and the filter each time. It really doesn't matter what type of oil you use if you change the oil and filter at least every 5,000 miles. I only go 3,500 miles but that is me. I drive hard and long distances in my work. Be sure to look at the weight of the oil when you buy it. the weight will be either a multi weight like a 5w30w or a straight 30w. look at the owners manual for the weight you should be using. In lawn mowers they call for a heavy duty 30weight.
Good luck. Hope I haven't been too much of a confuser. GB