I'd use 5w30 synthetic all year round. It'll be probably pretty close to the same and maybe even higher viscosity at operating temperature than 15w40. It'll have higher viscosity at startup temperature than it is at operating temperature, with 5w being lower at startup than 15w. On a JD oil application chart, for instance, 5w30 is shown to span the entire operating temperature range.
You have to see the viscosity values plotted with temperature to appreciate this. Viscosity is inverse with temperature. The higher grade (e.g. the xxw40 or xxw30) is determined by measurements of the viscosity at 212 F. Whether it meets the lower grade (e.g. 5wxx or 15wxx), measurement is done at a very low temperature, generally about 0 F. It used to be 100 F. Operating temperature is actually up around 250-350 F in terms of the actual temperature of metal that the oil is in contact with. Some engines have oil temperature measurements, which run close to the water thermostat temperatures. This is the oil sump though and probably the coldest thing the oil sees. So, the operating temperature is generally just above the temperature that the high grade value is measured at. If you have a lower low grade value, this will swing the viscosity of the oil at operating temperature even higher. So, to maintain near the same operating temperature viscosity, if you have a 5wxx oil, it should be lower at the high vis value, e.g. 5w30 instead of 5w40.
You can mix the oils, but putting just a little bit of dino oil in with synthetic loses some of the value of the synthetic. The synthetic properties/value will actually increase with each oil change that you do with synthetic because you'll flush more and more of the leftover dino oil out. Keep switching back and forth and you lose some of the benefits of the synthetic.
Ralph, retired lube process engineer
