A major oil company claims it is busting a dirty little secret of New Zealand motoring - that our petrol and diesel can cause dirt to build up in engines.

To counter it all, BP has added what it calls "Active" technology to its Ultimate brand of diesel and higher-octane petrol. This formulation, which is the result of five years research and first added to fuel in a trial in Spain, is designed to fight dirt, help protect against dirt buildup, and help engines run as the manufacturer intended.

Studies have shown that sooty or slimy deposits can build up on the intake valves of petrol engines over time, restricting their ability to "breathe", while diesel engines can be affected by a buildup of dirt deep inside fuel injectors or on the injector tips.

BP claims that with the Active technology, millions of molecules attach themselves to the dirt and drags it away, and also attach to metal surfaces on clean engines, forming a protective layer.

BP New Zealand's general manager of fuels, Debi Boffa, says dirt can clog important engine parts, rob a car of performance and make it feel sluggish, but what many people don't know is that it can also damage an engine over time.

"Our new fuel can fight this dirt and help protect against dirt build-up, which is pretty ground-breaking," she says.

A cleaner engine means a more efficient engine and compared to regular diesel that means some Ultimate diesel customers could get up to 56 more kilometres per tank.

The benefits of BP Ultimate Diesel with Active technology are compared to regular diesel, achieved over time and based on a vehicle range of at least 830km per tank.

The benefits claimed are based on industry standard drive cycles in a laboratory, may not reflect real world driving results and may not be attainable for all vehicles, says BP. Benefits may also vary due to vehicle type, vehicle condition, driving style, driving conditions, tank size and other factors.