The simple formula I came up with, based on analysis results of Amsoil in other applications, is to change oil when total fuel consumption for the drain interval =

600 gallons * sump capacity in qts/5 qts * cubic inches/HP

If we take my new Audi TT quattro, we have the following variables:

27 mpg average

4.8 qt sump

108 Ci

225 hp

600 gallons * 4.8/5.0 * 108/225 =

600 gallons * .96 * .48 = 276 gallons

276 gallons * 27 miles/gallon = 7452 miles between changes. So I'll test the oil in my new TT for the first time @ 7500 miles and go from there ....

The nice thing about this formula is that it takes into account variations in driving conditions. For example, stop and go driving and short trip driving in cold weather are very hard on the oil. These conditions also result in poor fuel efficiency. Highway driving at moderate speeds is ideal for fuel efficiency, and also promotes long oil life....

You can adjust the number of gallons you start with for other synthetic or petroleum oils and use this formula the same way ....

TooSlick