Hi Clarkie - I haven't replaced a chain in years actually. Though admittedly I am a fairweather rider these days (I wasn't always) and I am too embarrased to admit how many years it has taken me to cover the last 20K miles! Therefore my chains stay fairly clean and I just lube them as required.
If it works for you though I am sure it is fine - just sounded a bit odd as WD40 is so good at removing any trace of a lubricant.
I used to have a SP1 and 20K miles from a chain sounds mental .