If we're talking burning oil then Reagents would supercede this. Afterall it's a naturally occuring 'enhanced' burning oil. You could look at lesser versions of Greek oil and have them the same for Acids or corrosive airs.
The core rules Page 85 pretty much gives us a definaitve answer about this (paraphrasing a bit)
A jet of magical water that fails to penetrate means the target doesn't even get wet
a jet of mundane water that fails to penetrate means the target gets forcelessly wet.
If we talk about a mundane flammable oil being splashed over the magus, it would saturate etc and I would say burn as the creeping oil (CrAq 15) spell would. However IGNITING the oil once it had saturate the magus clothing would be difficult as I would say it had fallen into the catagory of being part of his clothes and therefore covered by his Parma.
The Firewood I would say that if the firewood was Mundane then once it is burning the fire is a mundane thing even if lit by magic. If the fire is somehow sustained by magic then it is a magical medium and resisted. If the Wood is not mundane but magical, then I would say that the guidelines on Creo are pretty good for this in that in that mundane food only nourishes for as long as the spell is in existance, but remains a magical medium (much like a magical flame or acid).
Curiously this suggests to me that whilst I am unclear as to wether burning magical wood is a footprint of magic or not (and therefore mundane and able to bypass MR) it does actually suggest to me that (non-ritual) magically created food and drink is resisted by the Parma.
A