One more thing to consider, how multi vis oil is made with dino vs syn.
Dino 10W-40 for example is 10 weight oil. They add long chain viscosity modifiers to get the 10 to behave like 40 when hot. The oil molecules are still really 10 weight. With use the viscosity modifier package is exhausted along with other additive chemicals.
By design, true synthetic oil behaves more like multivis. On both ends. It flows better cold so it can be thicker at room temp and still get the 10W rating. It also tends to thin less at high temps. Because of this, less or in some cases no viscosity modifiers are needed to get the multivis rating with syn. This also increases oil life as less or no viscosity modifiers to break down with use. They tend to be broken down during extreme conditions (motorhome duty).
In my opinion, based on cost to R and R a TZE engine, shorter oil change intervals are money well spent. Several factors go into oil change intervals, but one cannot be denied. Fuel rate and associated combustion byproducts. In rough terms our fuel rate is about double the Toronado (9 vs 17-18). That makes me think 1/2ing the oil interval is wise. Double fuel rate makes double the combustion water, though the high loads help to remove that water, the acids build up. Then we park it for months on end. A bad formula.
I’d try a true syn as Larry (and Dick Patterson) recommends. Some oils are syn rated as they perform like syn but are not syn. Also Dick recommends not exceeding 40 hot rating.