Rather than "mix and add," why not follow Groucho's idea and just use a thicker oil? Something like GC 0W30 gold or even Mobil 1 Extended Performance 5w30, which supposedly is thicker than the regular Mobil 1 5W30.
My '01 I30 loved Mobil 1 5W30 and 0W30 and seems to do well on the current 10W30 EP. It makes it clear, though, that it wants an oil under 12 cst.