Bill, I know you've done alot of work on engine oil and determining when it needs to be changed. I downloaded the spreadsheet just now, but I do not have Excel on my computer so I can't view it right now. I was wondering what you would consider an appropriate interval for changing engine oil. I use Mobil 10W30. I drive alot, 35K a year, and drive pretty hard being the leadfoot that I am. Also, and this may be answered when I can look at the SS, but what about the "dirtiness" that you see when you look at your oil when it hasn't been changed in a while. You know, looking brown instead of clear-gold and what not. Are these impurities non-harmful (since you say changing at 3K is a waste of $$$, yet my oil definately looks different at 3K than at 1K.) I'm at about 5K on this change, since I drive like 700-800 miles a week, putting it off for 2 weeks can really put me over the supposed 3K safe limit. Anyways, thanks for the reply in advance.