Total Seriola 32 is this suitable for both open and closed systems and what does "suitable for open systems" actually mean ? How is this different to a heat transfer oil that is only suitable for closed systems?

It is a good question. At Olieonline, we advise against using mineral oil in an open system. An open system means that it is directly connected to the outside air. For this reason, we do not consider it wise to use this oil, as it could cause a fire. An open system has continuous access to oxygen, which will also accelerate the oxidation of mineral oil.

We therefore recommend limiting the temperature in open systems: https://www.olieonline.co.uk/molyduval-silo-d-50