Hi Womble,
Does that UK manual definitely relate to the HZ engine? I would have thought it more likely to be the HD? I suppose it's irrelevant, the principles are the same.
Either way, the gist of it is that, for the 1HZ, Toyota initially prescribed oil & filter every 5,000km, and then later conceded that every 10,000km would comply.
I recall Tom Sheppard is a big fan of using oil with the highest possible TBN, which I did for years (Castrol Enduron Plus), until lengthy discussions with Castrol.
Choosing the right engine oil is a complicated business (unless you're just looking for lowest price), dependent not just on engine type, but its manner of usage.
On the subject of 2-stroke - remember the 1HZ is a 1990 design, back when there was plenty of sulphur to lubricate fuel pumps, injectors and everything else.
Add 500ml of 2-stroke per 100L of diesel. You will immediately hear and feel the improvement. The engine runs smoother, quieter and genuinely feels stronger.
I wouldn't worry about trying to calculate any fuel savings (whether real or perceived), the only reason for adding 2-stroke is in the interests of engine longevity.
Something else to consider are Forte products (Forte, not cheap imitations). Diesel Treatment, Diff & Gear Treatment, Motor Flush are all well worth looking into.
Other oils: have you changed your gearbox, transfer case and diffs? You should, and there is one oil you can use for them all - Castrol Syntrax Long Life, 75W90.
I appreciate there are those who will feel the above is all unnecessary and wasteful. I have no argument with them, but in my opinion, cheap is never cheerful.
Nor do I have any arrangement with Castrol, Forte, or anyone else for that matter. But I have spent many years researching, and sifting the shit from the clay.
All the best my friend,
Barrie