For NA engine, is there any advantage of using the oil effective against LSPI?

LSPI usually does not occur on the NA engine; however, since the new standard of oil clears old standards, there's no need for concern of using new oils. Even if the oils is designed to be effective against LSPI, other performance is not sacrificed, and the overall spec is improved by the latest formulation.
Super NA Racing is also available for the downsized turbocharged engines which engine oils is designated to be 0W-20 or 0W-30.

Why is the additive effective against LSPI not contained to Super Rotary Racing?

The rotary engine usually requires low ash property; therefore, the additive components level is usually low. As the compression ratio and boost pressure of the rotary engine are not so high, the proven conventional prescription was selected.

What is the advantage of the 100% synthetic base oil?

100% synthetic base oil can improve the fundamental strength of the oil. A strong oil film can be maintained without an additives that deteriorate by exposure to high temperature and high load conditions.
The additives work like an energy drink; the performance is temporarily improved, but the performance declines with the lapse of time. Therefore, the base oil’s fundamental strength is important to maintain the performance.

Why 10W-40 was selected for Super Boxer Racing?

Due to the short stroke, big bore, and aluminum block, the piston side knocking sound sometimes occurs when the engine is cold. Since the noise tends to increase if the viscosity is too soft, moderate viscosity was selected.

For RB, the viscosity of the former engine oil was 15W-55. Why is the viscosity of the new one softer?

The viscosity description was changed according to the standard for viscosity description.
There is a certain range in the viscosity index classified as 15W-50, and SUPER TURBO RACING 15W50 is designed to be thicker among the range.

Is there any disadvantage of new viscosity of new oils?

HKS oils are a multi-grade oils that is soft at a low temperature, and thick at high temperature.
Fuel consumption, startability, and oil film strength are implemented together at a high level.