When is it best to treat a car/truck engine, before or after an oil change?

Neither way is wrong. In new or almost new engines it doesn’t even make a big difference when it is performed due to not having many contaminants yet.

The higher the mileage, the dirtier the engine internals usually are. During the treatment procedure, dirt is removed from the surfaces and it gets to the oil. That’s why it is often recommendable to change the oil before the second treatment that we recommend for engines with mileage higher than 25,000 miles (40,000 km) Therefore the most optimal time for starting the RVS Technology® treatment procedures with a used engine is about  250 miles (400 km) before a scheduled oil change. The second treatment is then carried out after the oil change, and the RVS Technology® substance can be left in the engine until the following regular oil change.