WD 40 won't last long, and I wouldn't want the heat to evaporate the fumes into the food. Even if already a little surface rust, it really needs to be permanently coated with something.
You haven't yet stated what year your P is, use the code on the vent and check
here.
The handle on top of the tank is the first tipoff to the OPV issue: if basically a 3 sided handle, it is OPV, and the tank is good to the cert/recert date stamped into the tank handle. If a 10 or 12 pointed handle (old style), it is probably not an OPV tank - that's the quick visual check a refiller looks for. Although it
may be an OPV tank if there is a tag wired to it which states it is an OPV tank. A buddy of mine bought a '97 P, refiller wouldn't fill tank due to 10-12 point handle=non-OPV, until after about 10 minutes of discussion we finally realized the tag stated it was an OPV tank, then he happily filled it. So apparently there was a changover period to watch for...
If an old non-OPV tank, an OPV valve can be installed (don't know how much) by a propane technician, and the tank recertified for more years of service. Or, you can order one from Weber ($50-60). Or, others have gotten 5 lb tanks from (I think) Bass Pro Shops, Gander Mtn., Cabela's, etc.
If you go that route, remember to keep old parts, just for posterity...