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Lose of oil pressure

J

jclays

Regular Contributor
Took the boat out this morning to run her. She's just been doing low speed harbor cruises lately.
Went on a 20NM trip cruising at 2500RPM SOG was 19.8kts used 13gals of gas. All was normal temp, oil pressure 40psi Twin 350cui Crusaders on a 30ft 1966 Californian express cruiser. When I entered the harbor I dropped the speed to 700RPM to achieve the 5mph speed in the harbor. I noticed the oil pressure dropped to 20psi after about 5min the starboard engine rose back to 40psi. The port engine took about another 15 min to get up to 40psi. Never seen this happen before. 40psi is the normal operating range on these engines. Any thoughts. Oil change is due. Viscosity breakdown due to running her a litte hard? Oil levels are full.
 
Normal behavior for many BBC engines. Hot oil will have less viscosity, and build less pressure at low oil pump rpms. Also, there is some engine to engine variance due to bearing clearances, etc. Are you running 40 weight oil?
 
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Running straight 40w Valvoline. Should salt water boats run a marine oil?What is the difference? I've always run just straight 40w Valvoline. I was thinking of switching to Castrol GTX 40w. Only because thats all ive run in my cars and race car. Any opinions out there on oil for twin 350's salt water cooled living in the water 24/7 365 a year.
 
I've been running Castrol HD40 for many years. In spite of that, I recommend running a 0w-40 synthetic if you do any cold weather starts, and maybe even if you don't. I've started the conversion with Amsoil in one of my engines, and will likely do the other next. Use a high zinc loaded oil.
Ten years ago, I did a 454 rebuild, new bearings, but the main bearing clearance showed about 2 thou greater than spec. No harm, but hot oil pressure at idle showed 15 to 20 psi after a long (4 hours) run at cruise speed. Two years ago, a similar rebuild on the other engine, but the mains show right at the middle of the spec clearance. It keeps 35 psi or so when the stb engine goes to 15 at the same rpm. This has not shown to be a problem, but demonstrates to me the effect of main journal clearance on hot oil, low rpm pressure. THe engines are designed for 40 weight oil, when they were designed, the multi-vis oils did not last. Today's synthetic oils are much better, and should be used to provide better lube at startup/warmup. Keep the 40 weight, though, unless the engine is so worn that it will not develop 40psi at cruise.
 
I'm saying if you go with a dino oil, go with a straight 40 (better) or 30 if you will do cold starts. I'd also strongly suggest adding ZDDP if you do much low rpm running (trolling). Best is a synthetic 0-40, such as the Amsoil made for flat tappet engines.
 
I changed the oil yesterday to straight 40 weight, 15 lb's at idle, 50 lb's at high rpm. Much better then it was, thanks Dave and all.
 
Your boat's recent performance suggests that the change in oil pressure upon entering the port may have been caused by a breakdown in oil viscosity. You might wish to consider Castrol Aero 40 for your next oil change to maintain consistent oil pressure, particularly after running hard. It is renowned for being dependable under many circumstances.
 
With these GM marine engines you can use:
Straight 30 or 40
15/40
Merc 25/40
15-20 psi at hot idle after coming off plane is fine on these engines. You’ll see as the engine idles & the oil cools off it will increase back to 40 psi
 
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