Look up the rope material testing on the UIAA site.
Ah bollocks I'm bored so I'll help out
http://en.wikipedia.org/wiki/Dyneema and
http://www.dsm.com/en_US/html/hpf/home_dyneema.htm Maybe I should do a FAQ article on the subject?
Basically it's not true, petroleum spirits don't adversely effect rope/sling materials since they are veryresistant to such solvents. I read a while ago that rope testing of nylon ropes soaked in petrol showed no reduction in strength.
The killer with ropes and slings are battery acid leaks/contamination (which damage the polymer) acid damage has been linked with a couple of fatal incidents in the UK.
Basically spray with impunity, but don't overdo it. Too much will just attract dirt particles into the sling/rope which may abrade slghtly and oily ropes don't handle well.
3:1 (or GT85) will work better than WD40 anyway as they are lubricants whereas WD40 is primarily a water dispersant. Cam lube is a rip-off but comes in a nice bottle.