As I wrote elsewhere, I replaced the spark plugs on my 98 Legacy OB. I noticed that the old plugs wore unevenly, with #3 and #4 being in much worse shape than #1 and #2. Here's the picture.

 

Does that mean anything, or is just normal wear?

It could be a number of things really. From improper gap, to low compression. Hard to say.

Yes, that could be caused by a few engine ''problems'', including how tight the engine is in the compartment. It seems ''someone'' was complaining about that in http://www.ultimatesubaru.org/forum/showthread.php?t=106459 . Maybe only the front two plugs got changed the last time because the rears were considered too hard to get to by whomever did the work.

 

Well, that would make sense as to why the "rear" 2 plugs were practically gone. Haven't really thought of that.

 

That said, the plugs are all NGK's double platinums, so assumed that they replaced them all at the same time.