While the ZD30 engine should have been a major step forward in terms of technology with common-rail technology and double overhead camshafts, in reality, the engine was not one of Nissan’s finest moments. Bottom end failures were not uncommon with cracked pistons necessitating full rebuilds.

Several causes have been suggested including poor fuel-air ratio management and boost spikes form the turbocharger putting sudden, huge loads on the engine’s internals. Either way, though, the engine is far from remembered fondly.

CarsGuide has looked at this engine in depth, and even published a guide to the Nissan Patrol ZD30 engine.

Early 3.0-litre turbo-diesel Patrols were trouble, but the engines of the later years are not giving any trouble to speak of. It's important to check for a service record, changing the oil regularly is critical to a long engine life. Also check the diff bushes. It's recommended that you have your chosen car checked by a mechanic with knowledge of Patrols.

Read the report from Howard Ramsay and see you're not alone. The claimed fuel economy is 14.3 L/100 km for your model in urban, or city driving, but it the ultimate real life outcome depends heavily on the sort of driving you're doing. I reckon the dealer is probably correct, there isn't much wrong with your car.