A common question is which plugs should I use?
The common consensus is to stick to stock plugs on the 4.5l twin cam motor. The 3F is more tolerant of non genuine plugs, but what is more important than the plug type, is the condition of the plug.
An interesting post from James Douglass <firstname.lastname@example.org>:
After one year of very extensive testing of just about every type of plug in a wide variety of vehicles, there just isn't any measurable performance increase by switching from one readily available premium plug to another. Measurable performance gains were always recognized when changing worn out plugs to any new plug. That's how splitfire duped the public. A new $00.79 Champion plug will perform just as well as a new Splitfire. Some plugs will maintain performance longer than others but our testing was not measuring longevity only performance. There is however a new plug available in Europe that is not readily available in the US called BRISK premium. This plug actually showed an increase in HP and fuel economy from 2%-15%. The only drawbacks are that it is not a very good cold weather plug and the life span is about 15-20K miles. They are addressing the cold weather problem. The Brisk plug looks like no other plug. It does not have a negative post at all yet it produces three sparks simultaneously. Splitfire produces one spark that alternates between the two negative posts giving an allusion of two simultaneous sparks. Don't feel bad if you've tried them. I bought a set when they first came out also. I was involved in analyzing the marketability of the BRISK for the US but due to extremely high production costs it was put on hold. the biggest reason is that retail pricing would be around $15.00US each. If anyone has further interest in this plug contact me direct and I can put you in touch with someone who can get you more detailed info. By the way, I don't even know what kind of plugs are in my '91 80. After all of the testing we went through the most important thing I learned was that maintenance was important, type of plug was not.