After doing a bit more research, I can get an SAE 20 grade commercial diesel engine oil from my usual supplier. Unfortunately, the minimum order quantity is way too high, as it is usually only bought by commercial users. So, I spoke to the their technical man, and he didn't sound very enthusiastic about idea of substituting a 15w40 instead, as when it is warm it is substantially thicker than SAE 20 would be. The concern is not so much engine damage, but such a small low power engine might suffer economy/power loss that you wouldn't notice on a larger motor.
Instead, he's suggested I try 10w30, which is closer to SAE 20 when hot, but thinner when cold. So that's what I'm going to do. I'll keep a close eye on oil consumption. This is a bit of an experiement, so I accept I might need to change again if consumption becomes unacceptable.
In case you are interested, the brand I'm trying is called Fuchs Universal HD.