Originally Posted by jongies3
Unless you're running an air cooled v-twin motorcycle or old VW Bug in hot weather and traffic, a 50wt isn't necessary for a liquid cooled car engine, they'll never get hot enough to need the extra protection from thermal breakdown. 30wt is the universal average and most widely used viscosity to date for good reason, it offers good protection across a range of temperatures and is easy to come by. Try finding a 5W-50 at any parts store!
Many older italian, British engines prefer a 50 at operating temp, which are not air cooled, and specify 50 grades from the factory.
During the 60s and 70s, it was 20W-50 for almost everything in Europe.
These engine many times do fine on a modern 15W-40 HDEO but love 20W-50 too.