your machine needs class 15 bobbins. the "J" indicates genuine Janome brand. (i think. the J might also mean that the bobbins were made especially for Janome machines. or it might mean something else altogether, in which case i'm not only off-base, i'm running around the wrong ballfield. :lol: )
having read posts here for many years, and having heard some of the "advice" offered by dealers, i've come to the conclusion that some of them tend to overdramatize tiny details. who knows why. :lol:
any class 15 bobbin should work well in your machine, with or without the J.
most Singer machines with drop-in bobbin cases use a class L bobbin. if you sit a class L next to a class 15, you'll see that they are the same diameter, but the class 15 is every-so-slightly higher than the L.
the metal bobbin cases that you load and put in sideways usually work best with the metal bobbins with holes. they're the same size as a class 15 plastic bobbin.
if you put an L bobbin in a machine that prefers a 15, it will be a weeee bit loose, so will rattle around a little. most of the time that won't create problems. when proper tension is super-important, it's best to use the class 15.