From 241rwhp to 259rwhp with just a 3 degree difference? Well, from my understanding the oil should not make the pinging worse. Make sure your fuel filter is good, and that you run high quality 93 octane gas. I run the Bama X4 canned 91 octane race tune, and using 93 octane gas from a Race Trac station made it ping quite a bit so I had to retard timing 9 degrees. After running two tanks of 93 octane gas from a Chevron station I decided to play some with the timing and was able to run with only 6 degrees retarded without pinging. The more you advance the timing the greater the effect will be, but you have to be careful and run good quality gas and not something from your local ghetto gas station.
With the worn valve seals you will be fine just rebuilding them. If you have the money then by all means get the TFS Twisted Wedge heads, I have seen some serious power put down with those heads! They are expensive, but definitely worth it if your long term goals include a supercharger or turbo. If you stay N/A and get some cams you will stills see gains, but maybe not as much as you would like for the price.
I am just going to get my heads rebuilt when the time comes, my long term goals include a supercharger putting down 450-500rwhp on a rebuilt engine with forged internals. If that proves to be too expensive I will rebuild without forged internals and aim for 380-400rwhp.
2003 Mustang GT
Intake & Exhaust: K&N FIPK CAI, Mac O/R H-Pipe, Custom Catback w/ Magnaflow Magnapack Mufflers.
Suspension: H&R Super Sport Springs, KONI STR.T Shocks & Struts, J&M Caster Camber Plates, Steeda Bumpsteer Kit, UPR Elite LCA's & UCA's.
Performance: BAMA 93R Custom Tune, FRPP 4.10 Gears w/ Terminator spec T-Lok.
Planned Mods: FLSFC, Eibach Sway Bars, Throttle Body, Plenum, Rims.