I have 2018 3.5 Eco with the SP-550 plugs from factory. I also have a set of SP-534 plugs left from my old 2011 3.5. Would there be any reason I shouldn't use the 534's in the 2018 3.5? I had a look at the owners manual and it recommends the SP-534's and not the 550's?

 

I just did a change on my 2015 3.5 Ecoboost, I put in the SP-580s as they are supposed to be the replacements for the SP-534s. I imagine the SP-534s would still be OK, but the job is a bit of a pain so why not put in the SP-580s?

 

Although both trucks have 3.5L engines, I believe the '18 truck has a second generation engine as opposed to the '11 trucks first gen engine. I'd verify that the plugs are compatible before using the '11 plugs in the '18 engine.

 

Looking around, they changed from the 534 to the 550 mid-production at some point. Same plug, just the 550 is gapped 0.028" vs 0.030" for the 534. Maybe they found the plugs were wearing a little faster in the newer engine than they wanted so went with a smaller starting gap so they'd stay in range longer. To throw more confusion in, the current manual printing calls for SP-578 and gap 0.028"-0.031".

 

gap is only part of the equation. There are the thermal characteristics is well...what you see when people or descriptions mention hotter or colder plugs. You could have issues using a too hot or too cold plug just like incorrect gapping. There could be some differences there between those part numbers, not just gap. Even if so, those differences may not mean much to that engine’s normal operation which might be where the several different part numbers being acceptable comes from. Doesn’t have to be this part is superseded and renamed to that while still being the same old part kinda thing.

 

Didn't bother mentioning anything about heat range because as said, they are the same except for gap.
The SP-580 supersedes the SP-534. Those and the 550 are all heat range 1 as indicated by their full part number.
The SP-542 is the colder plug for a 3.5L

 

FYI : after 2011 Motorcraft plugs are now made by NGK (not autolite)
Something I've noted you will see Japan also printed on the newer plugs.....

 

Putting SP-580 @ .030 in my non-tuned stock 2016 3.5 Eco today. Based on all my research, these seem to be the appropriate plug.

 

Interesting, but not surprising. Autolite went to China like most of the others. No idea who actually makes them. Bosch plugs (even those marked with other country names) are actually made by Nanjing Huade in China. For at least the last 10 years, Nanjing Huade makes all the pieces and then ship them on containers to the "country of origin" where they are assembled.

 

Save a few dollars, lose thousands, be cautious. Have a warranty, be cautious? Manufactures usually don't change numbers for no reasons. Contact the plug manufacture, if they are the same brand for their advice.

Good Luck