My first Civic (Rover used it as basis for its 400 model) did around 190,000 miles and never a spanner on the engine and never used a drop of oil, mainly run on Castrol 10W40. My GD 1.4 Jazz did over 120,000 miles mainly 10W40 and 5W30 magnatec never a spanner on engine and used no oil, also used castrol gearbox oil with molyslip and gearbox input bearing never got noisy. My present Civic only done a mere 70,000 so far but would not be tempted to put 0W20 no matter what the price. As SparkyPaul says, any gain in fuel consumption will be marginal and with so many long established well known oil brands around for a few quid more than Petronus, the potential risks outweigh any gain.