that's their usual position, but its a bit of a load of crap.
burnt out valves are usually caused by carbon build up creating hot spots. combine this with the wrong fuel which burns as they're still open and it will burn them out.
but usually the initial carbon build up is caused by a lot of low speed city driving. take it for a good blat every now and then and run a cleaner through it regularly and it really shouldn't be a major issue.
i mean, you should use the octane rating its designed for as that minimises the potential, but in today's nanny state they can't say your driving style part of the problem and if you thrash the arse off it occasionally it'll be fine!
