I'm using Denso iridium for a cuple of months in a m60 without any problem. also I've been doing some tests and I think I'm saving about 1/2 km/lt (~1 mpg) @ 120 km/hr (75 mph). I bought it in www.sparkplugs.com. The plugs are expensive, $12 each, but if I'm saving gas, and here you pay $4.5 per gallon (half of the price is tax), I think it worth. Also, this plugs work for 30k miles.

Good luck,

Jose