Mirage with 158,000 miles which weight?

Pullman WA
I've got a 1989 Mirage with 158,000 miles. I've been using 10W-40 the last year or so with no problems, I've noticed that some people here have discouraged the use of 10W-40. My manual says I can use 10W-30 too, is it a good idea to switch? What really is the difference between the two oils? During the summer my maunal says I can use 20W-50 if it is above 32F outside (summer), is it a good idea for extra protection at higher temps? Thanks