Swarfy - you are right. Back in the day, engines used non-detergent oils: for engines little or no filtration, an oil that dumps out the muck as a deposit is actually the best choice. I've seen engines wrecked by people who wrongly decide that their engine will be better running on a modern oil. Why? Because modern detergent oils hold the muck and products of combustion in suspension to be taken out by the oil filter but, without an oil filter (such as our bikes) this oil keeps circulating as a grinding paste
You should use the oil the engine was designed for and just be aware of the places where sludge is likely to settle. For example, my Riley car, despite having a full-flow oil filter, is supposed to have the sump dropped every 10,000m to clear sludge. In our case, the problem is the crank acting as a centrifuge - a mechanism used on some cars as an actual oil filter - think it was some Renaults which had a centrifuge built into the crank pulley especially designed to filter out solids from the oil.
Sun's shining here today at last, so must get on with some outside work.
