If 2 spark plugs come out easily, and another 2 are a bugger to come out is that an indication of anything? I would of thought all four would either be easy or difficult. If one is hard to come out but is gradually turning (albeit with some force), do you just keep going till it comes out? Why would 2 be stiff and another 2 easy, as I presume they are all same age, but no idea how long they have been in. Can't see any danger of them snapping with a 14mm thick thread. I hope it is just old age and not undoing a cross threaded one. But I suppose undoing a cross threaded plug is not a reason for it to be really stiff to do undo?
Do people on here bother to use anti seize compound when installing spark plugs? Would engine oil do?Or that no-corrode jelly stuff that you can put on battery terminals do?