I am curious if my understanding of oil viscosity and use is correct. Volvo recommends to use 10W-30 or 5W-30 depending on the ambient temperature. I hear of many people using other blends and I believe many new cars use just 5w-30 year round. Is temperature the main determinant of the oil one should use or not necessarily? Do synthetics have the same stable ambient termperatures? Also, is it acceptable to mix 10W30 and 5W30 to come up with something that has a mixture of their temperature stability? Finally, I have seen recommendations that oil on turbos should be changed every 6 months irreguardless of mileage. Should time have much bearing on the change? should it not be solely mileage?

Thanks!
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Paul '89 740 Turbo 102,000