A word of caution on the oil and soap....it can be phytotoxic and you'll burn the heck out of most plants if you do it in sunlight; it is best done at dusk, dawn or an overcast day. It will burn some just on GP no matter when you spray.
There are products with out there designed to do the same thing, only better. They contain potassium salts of fatty acids and more than just coating the pest and essentially smothering it, they also penetrate the exoskeleton and disrupt the cellular activity. I use m-pede (which also has anti-fungal properties), but Safer Soap is probably the better known one, it isn't listed as a fungicide. My cost per gallon of spray works out to less than .20, so I get a better designed product than veg oil or pam and still not a drain on the wallet. Here again...the time of spraying is important or you get phytoxicity.
Another reason for their use when there is no sun is that they are only effective when wet, so sunlight can both result in burn and mitigate the activity of the spray.