I guess I'll be the voice of reason here, and leave the brand arguments to others.
First of all, where does he reside? Does he "need" synthetic oil, or does he just "want" synthetic oil?
What kind/type of OCIs will he be doing?
I too have an LBZ Dmax, and have seen stellar results of UOAs with traditional Delvac and Rotella. If long OCIs are not expected, and EXTREME temps are not anticipated, then your father can save a lot of money, and lose no ability to protect the engine, by choosing a quality brand name dino oil. by "extreme" temps, I mean well below zero degF, for most of the entire winter.
If he's absolutely convinced that synthetics are desired, then any of the three you named will perform well past what he's likely to put them through.
The RTS has proven to be outstanding for it's cost ratio. Perhaps the others are a tad "better" (depending upon how you define the word "better"), but I suspect the LBZ will never know the difference.
To many of us, we realize that there are ultra-performing lubricants. But we also realize that many times the cost/benefit ratio does not meet an acceptable ROI for our "normal" use.
If your father is going to be using as you've stated, and OCI'ing by the OLM or less than 10k miles, I suspect he'll be just fine with a conventional HDEO.
Still - many would argue about what dino oil is "best". IMO, any of the big three (Delo, Delvac, Rotella) are outstanding, and there are still others that don't garner the market share, but still perform with great ability, such as Tection Extra, Premium Blue, Long Life Gold, etc.
Typically, because I OCI with less than 10k miles each year, I simply purchase the least expensive HDEO I can find on sale from a quality brand name. The engine, and the UOAs, won't know the difference.
Discussing the minute differences is fun, but the reality is that it is moot in practical application.