This a recurring old theme with boaters that swears by them along detractors as well. Just posted my personal boating experience with good quality automotive gear oils. I'm not playing with fire nor hornets as well.
If you boat with new OB's that counts with fresh and flexible oil seals & o'rings don't need that marine stuff as won't see water intrussion any time soon, if boating with very old ones which you like using and buying as well as opposed to new ones, you definitely should. You should change as a preventive maintenence all lower leg seals from time to time to avoid failures, ensures no water intrussion which most boaters are definitely not doing due to a cost issue, time, what the heck issue, who knows.
Oil seals can fail any time due to : OB being very old and consequently oil seals too, OB seats for long time/year periods in heated closed environments or exposed to excessive & direct sun punishment while lower legs remains fully trimmed up for long time. in these cases, oil seals becomes dry and hardens, won't seal to perfection on prop shaft as a fresh one will. Water intrussion is fully guaranteed in the short run.
Why don't we start a gear oil poll to see how many Iboats users uses marine gear oils and which ones automotive gear oils, Just state OB manufacturing date as a reference.
Happy Boating