10W30, when the oil is 100 f it will pour out of the bottle like a 10 wieght oil and at 210 f will pour like a 30 wieght oil. Use manufacturer recommendation or a 5W30 or oW30 may be substituted for the 10W30 for better cold weather start uo and better fuel mileage. Going to a 40 wieght will decrease fuel mileage. Increase numbers when vehicles are using oil, like when you have high mileage.