Just my humble opinion, but...
It has been my experience that Bosch Platinum, or even Autolite Platinum plugs work absolutely fine for up to 60K miles in the SVX. I personally believe in changing plugs every 30K if for no other reason than to "read" & inspect them.
They are cheap, too!
Interestingly I have recently become a believer in just using the recommended type and heat range spark plugs in my '91 420SEL (regular copper-core Bosch) vs. any newer Platinum, +2 or +4, because when I changed back to the regular plugs THIS car started, idled & ran much better than it ever did with the Platinums.
