As already mentioned above, tarnish like that can build up for several reasons.
Cold running, not getting hot enough to burn off the moisture is the most common cause that can cause that, oil type can also contribute, ie low detergent oil.
Consider that if you see it on the dipstick, its also coated the rest of the internals with a thin film like that too.
So, you can either change your oil more frequently re how many months since last change, not on kms alone, or use a high detergent diesel rated oil to clean up the inside of the motor and then return to your usual oil with more frequent changes if doing low kms cold running.
Or just take the car for a longer run, so its brought up to operating temp for a longer period, giving it a flogging wont help it much, but getting it to operating temp and keeping it there will be more beneficial.
Cheers
T
Cold running, not getting hot enough to burn off the moisture is the most common cause that can cause that, oil type can also contribute, ie low detergent oil.
Consider that if you see it on the dipstick, its also coated the rest of the internals with a thin film like that too.
So, you can either change your oil more frequently re how many months since last change, not on kms alone, or use a high detergent diesel rated oil to clean up the inside of the motor and then return to your usual oil with more frequent changes if doing low kms cold running.
Or just take the car for a longer run, so its brought up to operating temp for a longer period, giving it a flogging wont help it much, but getting it to operating temp and keeping it there will be more beneficial.
Cheers
T