oil viscosity has two numbers ex: 20-50 weight. The first number is the operating viscosity when hot and the second is the viscosity when cold. Some oils are not rated at both extremes and are just ex: 30 weight. Running a higher hot viscosity to prevent oil loss is not a problem, also running a thinner cold viscosity in winter (up north) is not a problem either. Synthetic oil reaches "thermo-breakdown" temp at a higher temperature then standard oil. They also have chemicals to reduce the amount of moisture oils pick up during use. This moisture burns off during operation and creates hot spots in the engine and in the oil. - that's as simple as I can say it.