[svandamme]

It has a fan so nobody questions it being air cooled.

I don't think there has ever been pure oil cooled head,
anywhere, at least not for this kind of power out put per liter.
They all had fins for aircooling.

Going with oil cooled 4 valved heads does not make any sense to me
Porsche did water cooled 4 valved jackets on the 959 and several race cars.

Why would a small shop like singer do something that Porsche did not? for an extremely small productioin run.

Their clientele is not going to pay big money for a one off that uses prototype, untested technology .. They would not accept unreliability which comes with unproven tech.

the 4 valved water cooled heads are well tested, and reliable.
And they also have a scavenge pump for oil in the heads. That's the modern water cooled head for you.

Using oil for lube and water for cooling is better then using oil for both which degrades the oil's lubrication properties and increased need for filtration.