This calculator is designed to supply the area of any potential collection surface of any length and width in square feet and/or square inches, based on the length and width values entered in feet and inches, and the potential and estimated actual rainwater recovery in gallons.
It depends! Read the owner's manual for the brand of car you own or want to drive as each auto maker determines when to change the oil. Generally, I've read and experienced that full synthetic oil can last up to 5K-7500 miles, but... it also depends on other factors such as your driving style and where you live. VW has listed in its newer cars, like the 2015 VW Golf, that oil changes can be done in 10k miles!
Proper oil viscosity - thickness - is much more important under these operating conditions, but these conditions break down the viscosity more quickly. Therefore for the majority of cars on the road, a 3,000 mile interval for conventional oil and a 5,000 mile interval for synthetic oil are a safer bet.