I think many (most?) of us use a 20W-50 oil because its somewhat greater viscosity
might give a PSI or two greater oil pressure and because 20W-50 is the only grade that is likely to contain adequate ZDDP, known for its anti-wear and protective properties (particularly for cam lobes and lifter surfaces). Most new cars--I believe some BMWs are one exception--use lower vis oils since their engines are built with smaller machining tolerances and the thinner oils give a little bump in MPGs. The lower vis oils have less ZDDP since all new cars have catalytic converters--if they haven't been stolen--and ZDDP damages them. I don't know the whole story, but years ago my local parts house yanked all their Valvoline VR-1 20W-50 off the shelves and couldn't say why; that was about time California--it's
always California, isn't it?--was changing its requirements for oils due to the catalytic converter issue. It was back on the shelves a year or two later so some sort of compromise must have been worked-out.
You really can't go wrong with multi-viscosity oils, even if you don't live in a cold climate. A multi-vis oil makes for somewhat easier cranking and gets to the top end quicker. Some argue that since multi-vis oils start with a thinner base oil and add viscosity improvers ('VIs') to give the greater hot rating they will shear quicker and lose lubricating effectiveness, which is probably the case but I'm not a believer in extended oil changes anyway. My quad ATV, Gator, zero-turn mower, rototiller, edger, gopher gasser, compacter, Ranger and Lincoln get Costco's finest 5W-30 synthetic* (at less than $5/qt). The 2 ancient tractors get the cheapest 10W-40 I can find and my Mustang gets Ford's recommended 5W-20 semi-syn with my dealer's 'free oil changes for life' (although Ford does recommend a higher vis oil for racing, to be changed before and immediately after races). Some too-clever-by-half types decided 5W-20 was too thin and dumped 10W-30 and thicker oils in their Modular engines and broke their timing chain tensioners.
*
I look for sales and stock up.