[ecomodded]

I hear claims of higher mpg just by sticking a cold air intake on the motor, makes no sense, its really a warm air intake.. as it sits in the engine bay.
All cars get cool air from outside the car.
Most from the grill area near the headlight. My car the air intake is down the side of the fender pointed at the road, a Bad spot as it sucks the hot air off the road. Gasoline engines are reported to benefit from a "cold air" intake, diesels not so much, but that doesn't stop anyone from doing it.
I would assess your vehicles air intake as it is now and see if it is getting its outside air from a acceptable place if so then just put a clean air filter in.
My take on fuel additives is if they are for cleaning your injectors, there is products out that that work. Some diesel additives work that condition the fuel injector pump and its seals, so does adding a little 2 stoke oil to the diesel.
Slick 5o and other Teflon additives ? doubt it, but most have viscosity stabilizers that will rejuvenate thinned out oil, and will burn slower in a worn motor.

As for synthetic oil , its true ! it works the best and last longer before significant viscosity break down occurs.
My diesel is factory filled with synthetic motor oil, with a 20,000 km oil change interval.