Using 3rd party ink will require flushing the OEM inks out and using them. Than going through the process again to install the OEM inks, so you're technically wasting more ink and money all together. The head repair (out of warranty) is pretty costly and 3rd party manufacturer's don't make their ink exactly like OEM in which could damage the head quicker.
If you won't be able to get inks for some time, turn off the printer, cover it and unplug from the back so no current is going through it. An electric current can make the head act like a magnet and when dust and what not gets attracted to the metal in the head, will cause the nozzles to clog (one reason). Otherwise, it's the temp (to a degree). If you leave the printer on and change the timing feature from the 60 hours defaulted at, this will help the printer maintain itself better. The timer feature was integrated in the P series and has been doing a good job so far (per our customer reviews).
So, turn off the printer, unplug the power from back and cover with a sheet or something similar and you should be ok until you get new inks; if that makes sense.