I get the iridiums for 10 bucks (mates rates tho) at local auto parts place and highly reccomend them.
Seems they are recomended for LPG engines, so with all the LPG converstions in the last few years they have tacked on a nice margin.
I'm also useing OziexplorerCe, what a great alternative this is.
This is great value for money, with almost everything a $1500 unit does.
Except mine has a much larger display!!