In todays modern world of strict diesel emissions our engines are subject to more stress than ever. That is why regulary scheduled mainenance is so important. The addition of an EGR system to todays engines mean much higher ash content in todays oil meaning the oil is getting dirtier quicker. The stress that the engine and lubrication system go through during and active exhaust regeneration cycle is huge, more stress causes higher deposits in the oil and higher operating tempuratures mean quicker breakdown of oil.  The other side of todays modern engine emissions is that if the engine isn't running correctly and requires more frequent regerations then fuel dilution of the engine oil is highly possible and common. We often deal with fleets that run a lot of PTO time and don't drive many miles, don't be fooled by this common mistake. We see a tremendous amount of vehicles that have low miles bt crazy high operating hours, if this is your fleet it is time to start scheduing services around run time not mileage. The bottom line is that servicing your engine is always more cost effective than repairing it so why would you skimp on it?