If you are saying the M57N2 motor that is in the 335d doesn't have a dipstick, you might want to actually check out a 335d. Hint: look toward the rear of the motor on the drivers side for a red plastic grip loop....The N52 and N54 motors do not have the dipstick, regardless of model. The N57 diesel motor does not.
It doesn't sound like liters of fuel consumed is a reliable predictor of oil degradation. Conductivity certainly isn't a measure that I would gamble $16,000 worth of engine on. Used oil analysis to find the point just before the oil fails specification? Any day of the year. As others have posted, with a dipstick tube, it is real easy to get a sample of oil to send off for analysis.Folks, the oil change interval is simply a function of how much fuel is consumed. Each service interval is a specified # of litres of fuel; In the older E46 and X3, it's 2500 litres of fuel until the next oil change/Inspection.