Both of my tilt/trim reservoir leak, I think and are10 yo.. The reservoirs are held in place by 4 screws. I cleaned the top/sides/bottom of both before I go out. When I come back in, the top is wet and about 2 teaspoons of oil are on the deck per side. Don't see any leaks with in port operation. Took reservoirs off to check for cracks. Nothing.. O-Ring feels flexible.

I'm thinking buy a new o-ring and tank or am I missing something else?

Those things are the biggest pieces of **** ever. They all leak after a few years. Don't waste you're time, just put new ones on. What sucks is they aren't that cheap given their underwhelming quality.....

Is there a better aftermarket? Or Mercury Mercruiser is it?

There is: These Mayfair ones are pricey but nice.
Same page they have a $37.00 reservoir.
No experience as to the "quality" of it.
https://www.hardin-marine.com/c-766-...oir-tanks.aspx

This second link the first item posting is OEM through Hardin.
The second item is I guess their aftermarket kit they offer as a whole kit same as the Mercruiser versus just the reservoir in the other link.
https://www.hardin-marine.com/vsearc...hTerm=883166A2

FWIW, came across these 2 OEM kits at a good price.
1st link
2nd link

All over Ebay also: Here
Not really suggesting these. But I'd guess the Ebay $27.00 aren't any worse quality over the Hardin $77.00 kit.
Probably came over on the same slow boat from China from the same sweatshop.

I don't have hands on with these, but $300 seems like a nice price.

Fits-Mercruiser-Trim-Pump-865380A25.html

and I'm fairly sure they have a descent warranty.

I also moved to stainless brackets, I'm tired of paint.
https://www.ebay.com/itm/16570972783...3ABFBMhte50e1h

Very possible its your cap leaking and not the resiervoir. I've had to replace the caps before because of this.

I didn't check the caps. I'll check that this weekend.