18ft-lbf with digital torque wrench, for better accuracy.
With 4.7L (100 series, FSM is 13ft-lbf) I've been using Denso torque guidelines range, and picked 18ft.lbf for 14mm spark plug in aluminium heads.
But it's to soon to know, if this higher torque does much good, in stopping walk-out. I need to have some back in shop, to check at 30K, 60K, 90K, 120K, etc, To see if this stop the walk-out, slows or waste of time. But even miles is not a great guide. It's really based on warm-up and cool-down cycles (expansion & contraction) IMHO.
I notice with a Denso new plugs which has a fresh crush washer. As I reach ~17.5 ft-lbf with digital torque wrench. The plugs turns or slips just a tad, torque drops back a bit. I then continue to 18ft-lbf which feels very solid.
In 5.7L (200 series). I'd use 18ft-lbf also.
Here's a picture of set of replacement plugs (~50K on them) someone installed. Then ~5K ago, a well know Cruiser shop in CA, replaced head cover gaskets and tube seals. #1 tube seal apparently got hung up, cutting it and coming out top of cover leaking badly. #1 Tube was full of oil.
#1 (upper left corner) in picture. You can see how much oil after lightly wiping, still remained and soaking the paper sack. Coil is sitting to side (out of picture in pan) with oil dipping off.
Look at #5 (3rd down on left). That was a loose (walking out) plugs. The gunk on it's nut, is from combustion gasses blowing past the thread.
Coils likely switch around during head gasket job, by other shop. So likely #8 (lower right) coil was in different cylinder. Look how dark and burnt looking #8 coil is. That also is damage form hot combustion gasses passing threads. The brown coils boots, are also combustion gasses damage.
If engine running rich and plugs removed or after raising RPMs for 5 mins. Plugs removed that are wet, are sign of possible oil ring leakage/seepage.
When oil gets in spark plug tube, plug is dripping wet Note: if plugs loose, oil drips into combustion camber. S that oil will just collect there. The coil, if leak went bad or continued long enough, is also dipping wet. See how much oil dipped on brown sake from #1
Often times oil comes out the tube and covers head cover in that area. Combustion gasses will also, but typically not as wetting to head cover. Before remove plug, we see puddle of oil in tube, with tube seal leak.
Plug thread can be wet for other reason also. Like loose and running rich, oil ring, PCV clog, etc..