In designing for a wide range of service conditions, it's usually necessary to trade-off one characteristic for another so that the oil will perform at least reasonably in each application. Thus it's not likely that a multi-purpose oil or blend will be the best choice for any specific type of service. This can become a concern in applications where truly demanding service conditions must be met. Heat Transfer Fluids Multi-purpose oils are occasionally used in the transfer of heat, which is a particularly demanding application. As a direct result, a number of thermal fluid system manufacturers will not approve anything but specific-purpose heat transfer fluids for use in their equipment.
Some manufacturers even void equipment warranties if non-approved fluids are used.
Problems Reports from customers detail significant problems in systems running multi-purpose oils. Equipment damage has included failed heaters, stressed tube bundles, fouled valves, sludged pumps, and severe blockages in heat users. As such, they have the advantage of being cheap and immediately available. But some questions should be asked. Some do not.