Welcome!
Yes 5W40 full synthetic would be a perfect choice for oil in hot temperature usage. It should protect your engine well as long as it is from any high quality manufacturer and is rated for diesel engine applications.
I am not familiar with the Mitasu brand as it is not sold here in the US. However, it looks like it has a good quality reputation. My only question would be to note that they make two different synthetic 5W40 products, one of which is marketed for diesel engines and the other which is not. I'm not sure which one you would want to choose. The one that is not marketed for diesels does meet the VW 502.00/505.00 spec which the diesel product does not mention, so probably either one would work?
Diesel:
https://www.mitasuoil.com/en/mj-212
Platinum Long Life (diesel usage not mentioned):
https://www.mitasuoil.com/en/motor-o...-100-synthetic
You may also have other good European made options available for 5W40 diesel synthetic oils, like ROWE, Total, ELF, Pentosin, Lubromoly, Motul, Castrol EDGE, etc. Not sure if Mobil 1, Shell Rotella, Valvoline, etc are in your part of the world but they are common on our shelves and also good options. In short, as long as you use a good product in the right grade and formulated for diesel engine use, you'll be fine.
Looking very nice on those parts. Please share more photos of your rebuild project as you continue, we will enjoy seeing it!
Your picture shows two different valve cover gasket sets. Recommended is to use the one on the right side (the one-piece rubber type) rather than the four-piece cork based set on the left side of the picture. The one-piece gasket is much better, does not tend to leak, easier to install, and reusable.
