'99 F350 7.3L Power stroke - 180k miles - recently ( even in mild weather) it won't start without a shot of WD 40 in the air intake. Glow Plug relay tests OK. Once started, it will start fine all day, but if it sits overnight, needs WD 40 to start. About the time the starting problem began, had the passenger side fuel line replaced due to leak at the engine end fitting. Tranny rebuilt about the same time.

Was going to suggest to check the fuel lines. Its possible you have a bad o-ring along the way and when it sits overnight then it gets air in the lines.