They are different heat ranges. I think the 10s are a colder plug than the 8s. But I'm no longer sure!

 

Hey poco........I am not sure either, but I think the lower the number, the colder the plug.
If you hold the two of them together, the colder plug will have more insulation than the warmer plug.
Or, to put it another way, the air space between the insulation and the plug wall will be less on the colder plug. Can anyone tell us which is which looking at the two plugs?.............................pg

 

Pinion, after much time gazing at the lint in my belly-button, I think you're right.

 

hey poco......I was not sure, so I had logged on to a spark plug site somewhere and it listed equivalent brands of plugs with a heat range selection. In all cases the lower numbers were the colder plugs on all the different brands (that used numbers). Not too scientific on my part, but that is what I found. And then I recalled that in the old BSA Gold Star days, an N-2 plug was colder than an N-4, so that fit in.................pg

 

Here are two photos of an H8C and an H10C side by side. The H8C is the new plug in both photos. The only difference that I can make out is that the insulation covers more of the "metal thing in the center" in the H10C than it does in the H8C. Does this make the H10C the colder plug?

 

ChrisB.......I feel like I am ready to jump off a cliff here.... but here goes. The plug on the left (first photo) is the colder plug. The plug on the right (second photo) is the same plug and is the colder, again.
The reason for my choice is because the insulation on the colder plug covers more of the electrode than the more exposed hotter plug. Also, notice in the second photo that the hotter plug has the electrode extending further out into the combustion chamber area. This is the easiest way for me to denote a hotter plug from a colder one when I can compare. Very good photos of those plugs Chris and thanks for posting them.
OK, so who is going to be the first to blow me out of the water and sink my boat? Fire at will everyone..............pg