Dave...... glow plugs are like light bulbs.... they either work or they don't. Thirty seconds isn't a long time for a diesel of your vintage, and neither is the second or third trying to start before running, excessive considering the vintage of the tractor. I would say that it is normal from my experience. My 1963 Mercedes took 5 or 6 tries before it would fire up in cold weather, and my 1987 would start on the first try every time. Diesel engines have come a long way in the past 20 years and it isn't reasonable to put modern day starting standards on a engine that is of your engines vintage. The only thing that I would consider changing is the oil to a synthetic for better start up lubrication. If you had a dead glow plug, you would have a lot more problems starting that engine than 3 tries. There is a good chance that one defective glow plug would be enough to keep it from starting at all at the low temperatures you are experiencing. My BX22 took 3 tries today to start and that was with glowing the glow plugs each time for about 15 seconds. I consider this normal given the -4 degree temperature. When it is 40 degrees, it will start the first time every time with very little glowing necessary.