I've just rebuilt a small-block chevy engine (Chevrolet 350CI v8) for marine. It's been taken through the break-in with 5w30 oil, and now has 5w40 (mfg recommends 15w40, but I could not find that in a low detergent oil, I also run it in a colder than usual environment).

  • Journal clearances were 0.003", or slightly under. Somewhat high but well within tolerance for the motor (2.2" journals).
  • The engine does not consume oil, which makes me believe the piston rings are seated properly.
  • The oil filter is a wix ph5 - I cannot find the spec on the relief valve pressure for this filter, but I can't imagine it would be as low as 40psi. The filter is new

With the 5w40 oil, the pressure at cold is ~40PSI at idle, and ups to the 60s with a bit of throttle. When hot, the pressure quickly climbs to 40psi under throttle (anything over 2000RPM), and stays there. All the way up to ~3800RPM which is where I stopped advancing throttle, the oil pressure barely climbs, if at all.

Is this possibly a sender issue? It shows the higher pressures when cold, but even then, not by much - the pressure gauge seems off by 5-10 psi, because it sits about that far below zero when off, and sits at 2-3psi (but bounces around slightly) at a warm idle.

I have not tried a mechanical gauge as I don't have one, but will probably be picking one up today.

The engine sounds happy. Oil is clean, apart from the initial greying at break-in, the new oil is in very good shape. I have yet (but plan to) cut the break-in filter open to look for shavings, but based on the condition of the oil, I am confident it is okay.

1 Answer 1


40psi is adequate, if not good, oil pressure for a Gen 1 SBC. I'm not sure what the spec says for a marine application, but I'd suspect the 40psi is going to be well over what is considered "low oil pressure". In older automotive vehicles which had a SBC in them, the "idiot light" doesn't come on until the oil pressure was 5psi or below.

You really should be using the 15w40 oil with the increased clearances. I don't know why you are afraid of high detergent oil, as it is only going to provide you a higher cleaning rate in your engine. It isn't going to damage it. As was discussed in a previous question, 15w40 was formulated for diesel engines. You can check the API oil standards for it, but I would suggest it will either meet or beat any of the oils made for gasoline engines. 15w40 should boost your oil pressure significantly, unless the oil pump is bypassing the excess pressure.

The API CK-4 standard states:

It requires more from the oils in terms of shear stability, oxidation resistance and aeration control than its predecessor.

This is exactly what you need in a boat motor. Running your engine at higher RPM, under load, and for extended durations, requires more heavy duty oil, which the 15w40 should provide. There's a reason why the manufacturer has specified this type of oil, and I would bet this is the reason why.

  • All very good points. The move against the high detergent oils was to avoid any issues with break-in, otherwise I would happily be running the heavier duty oil. Shouldn't 5w40 and 15w40 behave the same when hot? Sep 6, 2022 at 13:49
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    No, not exactly. It's hard to explain and I'm sure I'd fail to explain it very well. Sep 6, 2022 at 15:56
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    If you can connect a mechanical oil pressure gauge in place of the sending unit, this can remove any doubt whether or not the oil pump is suspect.
    – F Dryer
    Sep 6, 2022 at 17:10

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