Lower Unit Lube on small older Johnson

I have a question regarding sy

I have a question regarding synthetic vrs. non-synthetic lower unit lube for an older 1961 5.5 HP Johnson. My local marina recommended the Merc High Performance (Synthetic) lube for my small engine. After reading the label on the back it says "don't use with a non-synthetic lube". I assume that it means don't add "make up" lube that is non-synthetic when levels in the lower unit need some more fluid. My question is: Could I go back to a more traditional lube (non-synthetic) if desired on the next or subsequent changes or is it better to continue to use the synthetic fluid? I ask this because I have used the non synthetic fluid in my larger MercCruiser without issues and have plenty of that fluid available to me as well as being half the price for future changes.
I would appreciate any/all comments regarding same. Thanks much.

"I would suggest to keep on wi

"I would suggest to keep on with the 'old' type cheaper Hi-Vis type non-synthetic lube. Will do no harm. However if You decide to re-seal the gear-case, that might be an opportunity to change. Old seals and new synthetic may sometimes create leaks."

Thanks Morten for your respons

Thanks Morten for your response. I did use the synthetic lube this weekend because it was recommended by the marina "mechanic" but will change back to the "old" type cheaper Hi Vis lube next weekend. I ran the engine with the synthetic oil for about 30 minutes in a test barrel. Do you think it was for such a short duration that the older seals would not be harmed?

"The seals itself does not get

"The seals itself does not get 'harmed', but the cleaning abilities of the synthetic oils may remove some old 'dirt' that helps keeping the seals from leaking. If the oil is there, just keep it. Hopefully no leakage. Just check for water in oil no and then."