706 gas Spark plugs

What is the correct spark plugs for my 706 gas tractor? I've seen people use Champions D21,D18,D16 as well as Autolites 3116. So I gues I would some info what is recomended by those of you who understand spark plugs and tractors.
Thanks inadvance
Matt

It kinda depends on what you are going to use it for and how good the engine is. The higher the number i.e. Champion D21, the hotter the plug. A good engine and heavy pulling, the D21 will cause the fuel to preignite since the spark plug elements will still be glowing red hot. This plug will do well with a lot of idling or an engine that is starting to foul the plugs due to oil burn.

Good engine and plowing? D14 or D16. Compromise, D18. Heavy pulling and backfiring thru the carb? Too hot, drop a heat range or two.

Champion D16 may have been the original recommendation when the tractor was new but it is also about the time that the recommended D16 was changed to a D15Y plug. After leaded fuels were removed from the market Champion changed the plug recommendation to D18Y for normal use. The D15Y can still be used for heavy duty, D18Y for normal use, and D21 for light duty only.

I'm reading from the pamphlet I got when I went to a week long training session at Hickory Hill, near Chicago in 1969. It shows the D-16 for intermediate service, 8-Com normal service, and D-9 severe service all as displaced champion spark plugs. The new approved champion plugs were D-18-y light service, D-15-y moderate service and D-9 severe service. A year or two later they introduced their low ash oils and valve seals and again changed their recommendations somewhat. I think the D-21 copper plus plug came along after IH was done making gasoline engines and it works very well for light to moderate service. Even with out low ash oils. What always surprises me is no one ever seems to even mention the 8-commercial spark plug which is what came from the factory in all of them engines. I personally had a good argument with a local parts store about that plug and the D-16. He claimed they were identical and I said they were not as IH classified them for different service. I then wrote a letter to champion and they verified that they were indeed slightly different but that they did not make the 8-com available except to OEM mfgr's .There fore all their books showed the D-16 as the recommended plug for the IH tractors. When you are burning valves and scoring pistons one is always looking for every reason and when you see these tractors coming in with the D-16 instead of 8-com you tend to make your own diagnosis. This led to a lot of whatevers and what if's and is another one of the reasons IH changed their recommendations to eleminate confusion. They also list this as one of the reasons in this pamphlet.