I have been advised today by the service manager of Norton Way Honda, Letchworth, that soon for all petrol engines they will be using Castrol fully synthetic oil 0W-20 as approved by Honda. Currently they are using Castrol fully synthetic 5W-30, which was put in my Accord 2.4EXi at recent 5 year service at 39000 miles. I also had the gearbox oil changed & car does seem to be using slightly less petrol. For my S2000 I will continue to use Shell Helix Ultra 5W-40 fully synthetic oil which I supply & Norton Way are happy to use without any problems being raised regarding three year Honda extended warranty with wear & tear which I purchased through them. This oil has been used in S2000 since its first service in 2010. Apparently there is some loophole which oil companies are using to describe their products as fully synthetic whereas most are not true "fully" synthetic. Any thoughts on the use of 0W-20 in older petrol engines?