"ACF50 is not good on air cooled engines I've found. It turns into a brownish varnish when the engine gets hot."
good point - I found the same thing. Nearly shat myself first time I saw it - thought something was well and truly gunged-up somewhere and was now finding its own way out!

The WD40 down the 4x4s seems like a good idea - cheaper than ACF50 too - and at the price for a replacement set, those pipes need all the preservation I/we can give them!
I sometimes think that maybe I overdo it with the preventative stuff, but then I'd rather do this than find out the hard way if maybe it would have been better to have done it in the first place.
... and it gives me an excuse to get down to the workshop! (again

)