My  NGK  plug was due for replacement, so I bought an E3 off EBay.
The E3 fit the threads in my engine and looked similar to my NGK.

After a few turns, my 18mm socket would not stay on the plug to finish  the tightening. The socket just spun freely and would no longer fit the flats on the plug.

I bought an 18mm socket from Ace hardware, because I was told their Craftsman sockets are thinner. Same result.

I bought an 8mm spark plug socket from Cyclegear because it was advertised as thin walled. Same result.

I lost patience and removed the stubborn E3. I put back  in the old NGK and my 18mm socket fit with no problem. It was easy to tighten the NGK plug.

It appears that something on the E3, other than the flats, was slightly larger than my NGK. maybe the upper part of the insulator was larger and prevented a good fit from the THREE sockets I tried.

I  plan to buy one of those cheap sockets like you get in  the factory tool kits. I can cut it down and customize one of those easier than a  nice forged socket, Maybe I will be able to use the E3 eventuallu.  

Has anybody else been able to use an E3 spark plug?

the spec sheet at E3 says it's a 5/8" not 18mm

16mm might work

I've got a lot of tools, but the first time I took the plug out of my S40 I discovered that I don't own a single socket that fits the flats on the plug,and isn't too fat to fit down into the recess in the head. The cheapo sheetmetal socket that comes in the OEM toolkit fits just fine though

Not sure why you want to run the E3 plug instead of a trusty NGK, but if you must, why not go buy a 5/8" spark plug socket?  They're cheap.  I have one and it fits in the cylinder head just fine.  Versy says the E3 spec calls for 5/8.   A 5/8 plug socket should work.