I went to buy a new spark plug for my wr400-99 & was shown 3 different types made by NGK. The prices went from $12 to $21 Australian dollars. The cheapest plug had 1 earth electrode. CR8E. The next one had 2 earth electrodes. CR8EK. The most expensive one had a center electrode made from RIDIUM. Has anybody used these plugs? Is one better than the other?