Oil choices...

I searched but didn’t find definitive info so I’m hoping for a solid answer here. 1986 4.3 omc cobra. Looking for recommendations on oils for the lower unit as well as the engine. Thinking of synthetic but can’t seem to find any info that says yay or neigh for any specific reason. Cost isn’t my concern, reliability and reduced friction is... tia

I’ve had an ‘88 4.3 Cobra for years and use Mercruiser for both:
25/40 Mercruiser/Quicksilver conventional motor oil (no need for syn in these old engines)
Mercruiser/Quicksilver High Performance gear lube GL 4 rated

both are widely available everywhere marine engine parts are sold....

Thank you!!!

I use 85wt gear oil in the outdrive, and the engine I use delo 400 diesel engine oil... it has superior protection over standard automotive engine oils, and is a better oil than semi-synthetics....

as far as full synthetic oils go, their main claim to fame is that they perform better at extreme high temps that some engines experience.... in marine engines that never exceed 200 degrees, there is no point in using them.... some people believe they offer something at the lower temperature levels, but this is false information given by some manufactures as a selling point.... yes, it offers good protection, but no better than a dino oil unless there are regularly sustained high temperatures.....

if using an automotive oil, I would recommend using a Zinc additive in the oil, which is what "break in oil" has in it, as the good lubricity properties of zinc has been removed from regular automotive oils, which are needed in older engines. modern engines with roller cams have less issues with the lack of lubricity that zinc offers...

if your engine never exceeds about 170 degrees, a 10-30 or 20-30 wt oil is good... the higher temp engines, (or worn engines) may benefit from a 15-40 oil, but offers little for a low temp engine like a smaller boat engine...

My interest in synthetic oil arose from the fact that this motor was recently rebuilt and I was looking for a product that would adhere to parts better to aid in protecting from dry starts which synthetics do offer. I just didn’t know if it had hydrophilic tendencies or not.

Mobil1 full synthetic 15W-50.....for decades...in the crankcase....

If you can find the Evinrude HPF Pro gear oil it is also very good, maybe better than the Merc High Performance but the Merc/Quicksilver stuff is also very good and everywhere.
Motor oil, well this has been debated for ever. If you have a flat tappet engine and a 1986 probably is, using oils higher in zinc helps. There used to be a lot more zinc in both Delo and Rotella but that is no longer true; due to concerns with diesel emissions. The last time I used Delo I had it analyzed and it was at 1700 ppm zinc but the current formulation is much lower. Look up the specs yourself . If the engine has a roller cam (my 1988 4.3 does) then it is not such a big issue. In fact when I did a top engine overhaul 3 years ago I took out the lifters to check the cam lobes and they and the lifters looked brand new. I think your normal sterndrive or inboard that doesn’t have an engine oil cooler should run higher vis oils like Mercs 25/40 or a 20/50. You want answers? Run it a season and do a Blackstone or similar oil analysis. That will tell you if the oil you used is good enough!