Have you checked the owners manual? An internet search produced these links to a manual.
https://www.manualslib.com/manual/927945/Yanmar-Ysb8.html https://forums.sailboatowners.com/attachments/yanmar_sb8_manual-pdf.216306/ The chart will help you select the best oil. You should chose the oil based on the highest air temp which you expect to operate in since you're unlikely to change the oil as the seasons change. I expect that for most US that would be 20-35°C(68-95°F) - Unless you are very far north and never expect to operate on a 68°F day that puts you using SAE 30. This is great news since as you've already found SAE 20 is difficult to find. Shell Rotella T1 is a highly respected oil and readily available. Based on the information in the manual the engine and gearbox take the same oil which is also nice since you won't have to keep multiple different products. I'm a fan of Amsoil products. Their tech support is phenomenal. I've had great success calling them for product recommendations when nothing else seems to fit the bill. A quick refresher in oil tech: 5w-30 = 5 winter weight and 30 straight weight. 5 means that the oil moves like an SAE 5 at winter temps. Once the oil is up to temp the oil moves like an SAE 30. They do this for quick lubrication on startup. It is done be incorporating polymers that "coil up" in cold temps and uncoil/get stringy at hot temps. But what it also means is that the oil is built from a lower viscosity (near SAE 5) then add the polymers. And the first thing to break down and burn up is the additives so the oil will slowly move from SAE 30 at operating temp to SAE 5 at operating temp as the oil gets old. This is both a reason for and against using multi-grade oils in various applications. All the best, Josh Muckley S/V Sea Hawk 1989 C&C 37+ Solomons, MD Oct 20, 2025 14:16:53 James Hesketh via CnC-List <[email protected]>: > When I got my boat with a Yanmar SB8 there were a couple quarts of 20w oil > for the gearbox onboard, which I've used. > > Now I'm looking to purchase more and the only straight 20W I find (on Amazon) > is a "Bearing Assembly Oil." > > Is that what I need, or what do people recommend for these old gearboxes? > > TIA > Jim Hesketh > "Whisper" C&C 26
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