My Hyster S-60 is maybe 10 yrs old, 10k hrs, wouldn’t start at all at under about 90 degrees ambient temp., but started fine with about 3 sec. spray of any starting fluid into opened air cleaner. Repair guy said it was the converter and installed a seal kit made for that converter. Then it started fine as long as weather was warm. Now that ambient is always under about 65 degrees, again it won’t start without starting fluid. A friend recommended Gumout brand as easier on engines somehow. What is a safe-on-propane-fueled-engine starting fluid if you have some technical rationale?