my new to me 09 superduty v10 has 50k on it now, how often should you change the plugs? some people tell me every 50k. What plugs are best?
Mine has 5 star tuning if that makes a difference on plug life.

 

90k is the normal spark plug change interval on the 3V Modular engines. This is from the '09 Scheduled Maintenance Guide:

90,000 miles
❑ Change engine oil and replace oil filter
❑ Inspect automatic transmission fluid level on
vehicles if equipped with dipstick. Consult
your dealer for your particular requirements.
❑ Replace cabin air filter (if equipped)
❑ Replace climate-controlled seat filter (if equipped)
❑ Replace engine air filter
❑ Replace fuel filter (except Escape, Mariner,
F-Series [non Super Duty],
Expedition/Navigator [short wheel base only],
E-Series)
❑ Replace spark plugs
❑ Inspect steering linkage, ball joints, suspension,
tie rod ends, driveshaft, and U-joints (lubricate if
equipped with Zerk fittings)
❑ Inspect engine cooling system and hoses
❑ Inspect brake pads, shoes, rotors, drums, brake
lines and hoses and parking brake system.
❑ Inspect half shaft boots (if equipped)
❑ Inspect exhaust system and heat shields
❑ Inspect accessory drive belts
❑ Inspect and lubricate 4x4 front axle U-joints
❑ Rotate tires, inspect tires for wear and measure
tread depth (vehicles with dual rear wheels
should only rotate if unusual wear is noted)
❑ Inspect the wheels and related components for
abnormal noise, wear, looseness, or drag
❑ Perform multi-point inspection (recommended)

 

I always changed mine at 50k intervals. If you wait longer than that you run the risk of breaking a plug trying to get it out. If you search about plug issues, most if not all the time people waited too long to change plugs and they had issues. I would never use anything but Ford Motorcraft plugs. Too risky using off brand plugs. Since you have a '09 you have the one piece plugs, you can change them out in 90 minutes.

 

Do not go 90k to change your plugs. I highly recommend the 50k changing, otherwise you stand the chance of having a plug go south and burning a hole in your piston. I waited until 92k and it was almost too late. Now it's every 50k I'm changing plugs.