Ah, the old oil question. ST 30 is a mineral oil with very little in the way of additives. It might be now considered a poor lubricant compared to modern oils. However it depends what you are using the oil for. Early multi-grades were an effort to harmonise oil grades into one package rather than differing versions of differing grades at differing times of year. Fine. Modern multi-grades are more technically advanced and are made for engines that run as modern engines do with the idea of pump lubricated shell bearings. So it is sticky to coat surfaces. That is why modern Multi-grade oil is not right for your Heinkel. This is splash lubricated on a roller bearing big end. If the oil is splash resistant and sticks to the roller-bearing big end inhibiting its ability to roll you effectively have poor lubrication and an early failure. As proof, all the people who used to belt about UK and Europe using modern oils and arguing the better oil meant longer than 600 mile oil changes and who promptly had engine failures most trips. Where as Mick Lesson and co, who stuck to the recommendations, had faster cars that lasted a lot longer.
Not knowing much of Villiers engines I cannot really comment but with cork friction material sulphur based, Hypoid containing oil and other additives strip off the friction material. By definition a wet clutch has to still have some friction to work. So too good a lubricant is, perhaps, a problem. I am less sure about the effects of a very sticky oil. It might effect release. Good for the chain? Effectively though the Sachs set up is made for crap oil technology. Maybe Villiers is the same? ST/SAE30 oil is cheap though. Best source, farm shops, as it is farmers light utility oil of choice and used on anything thought ought to move but might not, plus a lot of mechanisms such as lawn mowers.