I would say it depends upon the engine's state of repair.
The 45's are a step hotter than the 44's. Forty-five's are middle of the road heat range with 46 the hottest and 43 the coldest. I have seen 42's.
Most engines run a 44, however, depending upon age and oil seepage into the combustion chamber, a hotter plug may be warrantied. This hotter heat range will keep the electrodes cleaner within reason. If the engine is burning a quart of oil, say, every 500 miles, then, no hot spark plug is going to keep itself clean to fire.
Its trial and error, see what the book suggests for the year of block, if its 44's, try them out. After a couple thousand miles, pull the plugs to inspect the electrodes. If they are anything but tan in color, then change to 45's.
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