I think oil_film_movies is on the right track. One doesn't necessarily need to go up to a 40 or 50 to get a thicker oil than the typical GF-5 5W-30.
XW-30 can describe anything from Resource Conserving oils with a HTHS of 2.9 to ACEA A3/B3... and heavy duty engine oils with a HTHS of 3.5 and higher.
...i can bet outside the us there is a temperature graph where 20w50 is acceptable.
Couldn't find the 1MZ, but here's the chart from the owner's manual of the Australian Toyota Kluger
(our Highlander) with the 3MZ (mostly the same as the 1MZ VVTi).