spark plug

I went to change my spark plugs today and there was a decent amount of oil is the spark plug valley. I am going to clean up the oil prior to removing the plugs but was wondering what I can do to prevent this. I read a bunch of threads on this and felt like half of what I read was not correct...

Does anyone know why these Autolite 3923s are different? I bought 4 of these at 1 place and 2 at another place. I was going to install them today and realized they are not the same. One is slightly longer than the other, the metal is different and the end where the spark plug wire connects is...