I have used AL103, AL104, TR6 in the regal. I have used AL606 (came with the car) and TR55, once TR6 in my GP. I have used APP606 in my friends olds 88 L36, and APP605 twice in 2 different SS Monte Carlo L36's. TR55 for years in my old car 91 olds ciera, and my friend's 93 olds ciera both with 3300 engines.
here is what I have noticed:
on stock L36 engines putting the TR55 in messes up fuel trims making them around -7 at hot idle. Gas mileage seems unaffected from the stock heat range. And the TR6 on a close to stock L36 causes O2 sensor codes.
on my regal gs I didn't notice any difference from TR55 or TR6 vs. AL104 from a performance aspect. Starts runs as it should. With the AL103 I did have slight fouling occurring because it is my daily driver and I live 5 mins from work. What I mean by slight fouling is nothing noticeable driving the car, but from the tailpipe it would squirt a quick black puff on WOT and it would go away. I pulled them out after a 15 min drive and 2 hr cool down and the 103 had alittle carbon on the porcelain insulator but for the most part the plug was clean. I did the same with 104's and they were perfect so that tells me alittle too cold for my mods and driving habits. I also saw around 2-3mpg better on the 104's vs the 103's. This is all of course before purchasing a PT. I haven't put the 103's back in to tune fuel trims to see if this is what almost fouled them. I can say that with the 104s and my mods that I did have to lean cruise and idle trims to get them more acceptable -5 to 0.
mods on the regal are standard bolt ons. 3.4, PLOG, 3'' DP, stock cat, 3'' catback, wizairred.
conclusion, the 104's for the price and similar performance to the NGK's I will stick with them on the GS.
