Curious about spark plugs

Hi AF buddies, two years ago I bought this Lumina APV '91 and, being a novice with car maintenance, all I've been doing is the usual change of air filter, fuel filter, oil filter, oil, spark plugs, etc. (just only once I gathered the courage to make a change of fuel pump followed by a proper tank wash :naughty: ). But what's been boggling my head is: Why everytime I change spark plugs, SP#1 at the back seems to be the one that gets most of the sludge. SP#3 and SP#5, also at the back, get it at a lesser degree while #2, #4 and #6 at the front look cleaner than the rest???

Please illuminate my mind!!!

Your number 1 cylinder is probably burning oil to some degree and the 3 and 5 are not burning as much.
Is the engine a 3.1? how many miles?
Wife has a 91 lumuna APV but I think the engine was changed before we bought it a few years ago. Can't determine the age.
Put some oil additve in the crankcase that removes carbon and sludge then after a week change the oil again. Sometimes this helps frozen rings and valves.