Don't use engine additives
Engine additives glazes the bore or sleeves of the block. This results in ring slippage and lost compression - 'blow-by' past the rings. Don't add it to the differential or gearbox either, it makes the gears brittle and thus prone to break. Replace gearbox and differential oil every 30000km with the lubricant specified by the manufacturer. Some Transmissions/Differentials use a SAE90 gear oil. Isuzu TD280's gearbox must take SAE30 oil. Other manual transmissions use Automatic transmission fluid, such as the Borg-Warner (Tremic) T5OD transmission. Engine additives is probably the biggest consumer fraud in the automotive business. The only thing in your engine should be the relevant viscosity oil - full stop
A certain company that sells an engine additive has the following statement on their webpage:
XXXX Engine Treatment successfully reduces friction and heat in your engine with its patented technology.
Of course their product reduces friction - by glazing the bore. There has to be a certain amount of friction between the rings and the bore so as to prevent blow-by past the rings. A car plant hones a diamond like pattern on the bore in a deliberate attempt to create friction between the bore and the rings. This friction must not be reduced, which is what engine additives do by bonding with the bore. Each engine has a specific amount of friction that is measured by pulling out a ribbon gauge between the skirt of the piston and bore. The sleeves of an engine block experience severe damage from prolonged use of engine additives. If the damage becomes severe enough, it may damage them beyond repair, in which case new sleeves have to be fitted.