Agreed, with the exception that synthetics help maintain a cleaner engine and resist extremes (pressure and temperature) alot better than conventionals.
But you still are throwing money away if you only use synthetics for 6000-7000 miles. I'm over 14,000 on my run of AMSOIL SSO 0w30 right now.
Of course, it's like $9 a quart....so it better extend my intervals
Joe