seriously, this is really big matter.. most of our people doesn't know about it at all, just the item claim can save fuel, they just buy it.. no research, review it first, testing the product in details, and less reading ler...
in anything matters, the more u know, the more u dont know actually.. but better know something about it compare u know nothing about it..
so yes, leaner engine has lesser ur petrol usage, but it increase ur engine temperature.. so u know ur max engine temp, then u make until that temp n maintain it with good cooling engine system.. by doing this, u get more power n lesser usage of petrol.. sure can get more power with less petrol, but can ur engine stand with very high operating temperature.. so that's the prob for current engine, still no material that can withstand until 1000 degree celcius for very long period of time..