That sounds like rubbish
Most of the diesel oil sold on the market are 15w40. I believe Toyota uses Castrol Crb turbo or whatever the new name for it is which is a 15w40. The owners manual lists 5w30 as being preferred for good fuel economy and cold weather and also gives you a range of oils you can use; among which is the 15w40.
It also only requires an API CF rated oil which means that you can use most gas engine oils as most are rated to API CF
The Mobil is decent but it is a touch on the pricey side. It isn't necessary but if you want to use it that is up to you.
So far I have used.
Total quartz 9000 5w40
Shell Rimula R4 15w40
Chevron Delo 400 le 15w40

Once you changing oil and filters on time you should have no worries