And yet somehow, millions of Slant-6 engines without this year-specific problem don't run lean in 1 and 6. Huh. Habbout that. Lard works in mysterious ways, I guess.
Oh, hey, wait a minute, you're in Australia! So this must be one of those things like how water spirals the 'wrong' way out the sink and clocks spin the other direction, the Slant-6 leans to port instead of starboard, V8 engines have the intake manifolds on the outside and the exhaust on the inside of the vee, etc. But the OP's up here, so…