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I'm trying to source 15w40 oil for a marine engine (indmar monsoon 325 - a marinized version of a centerbolt SBC), however I'm not finding any conventional oils at most automotive stores near me. They all have a selection of 15w40 oil for diesel engines, including in big drums which is fantastic for me as this is a rebuild and will be going through many oil changes in the first few hours of its life.

The oils I am seeing for diesel do meet gasoline engine specifications as well, however they are marketed primarily for diesel engines.

Is there a fundamental difference between these oils? I know I need conventional oil for break-in, my concern is that diesel oils will be mostly synthetic as they burn hotter, whereas a marine engine on the opposite end of the spectrum only runs at 160 degrees.

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  • What is pushing the 15w-40 weight? You're right in that the weight is specified for diesel engines. The primary difference between gasoline and diesel engine oil is the additive package, where gas engines normally burn cleaner. Diesel engines are a bit dirtier, so requires more cleaning agents to contain the carbon. I'm not quite sure where you get the idea a gas engine runs cooler than a diesel engine. Diesel engines are much better at converting the heat energy into power. Gas dispels a lot more heat through the tailpipe, oil, and cooling system. Commented Aug 17, 2022 at 17:08
  • As an aside, if I were you, I'd just get a good break-in oil and go with it. A lot of them are a straight 30w, but also contain zinc to help the break-in process. Commented Aug 17, 2022 at 17:14
  • @Pᴀᴜʟsᴛᴇʀ2 this is a marine engine, indmar (dealer who bought a 350 and re-cammed it) specifically recommends 15w-40. Almost all inboard ski boat engines spec an oil this heavy. As for running cooler - this is also a marine thing, not a gas engine thing. The thermostat on these engines has them running at 160, as opposed to the usual ~212. Also - it's roller cam, not flat-tappet. Is the zinc still recommended? Commented Aug 17, 2022 at 17:19
  • Fair enough on the weight. Considering it's a roller cam, break-in should be real easy. You'll definitely need conventional, but not necessarily with the zinc additive. You may never get the rings to seat if you run a synthetic. If it were me, personally I'd just run a straight 30w conventional through the first oil change, then roll with the 15w-40 synthetic, such as Rotella or what have you. As I stated, the 15w-40 is spec'd as diesel, but it's not going to hurt your engine after it's broke in. Please note: this is just my suggestion. Commented Aug 17, 2022 at 17:31
  • Thanks for the help! To answer the underlying question - the diesel oil will be fine, but has a different additive package than the equivalent gasoline oil Commented Aug 17, 2022 at 18:31

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(From comments ...) The major difference between oils commonly used in diesel engines and those used in gasoline engines is the additive package. The base oils these are derived from (given synthetic, semi-synthetic, or conventional are fundamentally different) all come from basically the same thing. It's how its treated after the fact with the additive package which really makes the difference.

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