Technically speaking, what damage can be done if an oil thins out when in use? Let's just say for argument's sake that a particular engine likes an oil that is around 13cst at 100c. So you run an oil like GTX 10w40 and it starts out at around 14 cst and after 3000 miles it finishes up at around 12cst. So it's thinned out considerably, however it's still in the "sweet spot" of viscosity that this particular engine has been proven to like. So what is the problem with this scenario? Would this engine then have a tendancy to get more dirty due to the breaking down of the VII? Or is there really no harm in an oil thinning out as long as the viscosity is still within an acceptable range? One of the reasons I ask this is because of the recent rash of 0w40 results where they have thinned out, and also because of the recent LS1 analysis with GTX 10w40 where it thinned out a lot but the wear numbers were the best LS1 results I've seen to date.