Extending oil changes is a hotly debated subject here you might know, especially if it involves using a high-grade synthetic to do so.

Oil formulations and additives aren't all compatible, although there is some merit to replenishing some of the oil's additive package to do so.

The problem is that depending on what oil and filter you're using, the condition and tune of your engine, what fuel you burn, and especially how you drive; the condition of your oil will be vastly different.

For example: If you're running a '90 300D in great tune, no toying with the ALDA or rack, use premium or a good synthetic, OEM filter or equivalent, don't run it too hard and typically drive 30minutes or further your oil shouldn't be getting much soot, much un-burned fuel, or much water/condensation.

If you're running a 240D that is pretty worn out, drive a lot of short trips, run it hard, smoke a lot and burn cheap fuel and oil, you're just the opposite.

The oil breakdown includes pollutants: acids/moisture/fuel/soot, some of which can be filtered out and others can be neutralized by the additive package, the DieselService product can help there. Too much additive however isn't always a good thing. Other issues such as viscosity breakdown can't be repaired, the oil is just shot. Moisture and fuel will pollute the fuel, long trips and a good starting engine will minimize it.

The effective life of the filter must also be considered, most who run extended drain intervals agree that a standard filter should be changed at standard intervals even if the oil isn't.

Some oil additives (ex: IMO Slick 50) can be harmful to your engine, consider that the breakdown temperature of PTFE is lower than the combustion temperature in the cylinder, so not knowing what is contained in the additive would eliminate it from my list. If you're not comfortable with what you're adding to your engine, don't.

Good oil and filters are relatively cheap maintenance.

- Jeff
190DT:
Amsoil 3000 + .5u MoS2 Engine
Zerex G05 + RedLine Water Wetter
Mobil 1 ATF
Mobil 1 + MoS2 Differential
Lucas Low Moisture Brake Fluid
M-B Filters