Ok BEFORE your blood pressure spikes, and you have a BITOG induced aneurysm, It is noteworthy that Volvo Penta recommends 10w40 for this engine! But I sure caught heck on a boating forum I go to, so for peace of mind, I need some reassurance.
I have twin 4.3 EFI engines in my boat, about 350 hours total. Last season I used Valvoline White Bottle 10w40 - the only reason is because my Dad passed away and I found 2 brand new 5 qt jugs of it in his vehicle. Why he bought it, I have no idea, but either way, I ran it last season. The season before that I ran Rotella T5 10w30.
Last fall, I scored a good deal on T6, after rebate. I changed the oil in the fall and put the T6 5w40 in the engines. On the other forum I frequent, they are saying it will shear like a mofo. But I didnt just haphazardly buy T6 "just because", I researched it first and saw that it is especially shear resistant.
Penny for your thoughts?
Rotella T6 5w40 in a 4.3EFI Marine Engine
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No reason why that wouldn't work just fine.
That engine has been/is being used by several marine drive makers and T-6 is a thin 40.
Diesel oil is tough oil.
I run a 20w50 Amsoil in my Merc 454, but I am considering taking it down to 10w40. Most likely you have oil coolers, so a 5w40 should be fine. Try it, and at the end of the season run a UOA. I bet the room to work on those twin 4.3 is sparse ?
I ran T6 in the 454 engine of my old SeaRay for 135 hours, and the oil didn't even know it had been in an engine.
My wear metals were less than the universal averages for 48 hours.
That engine was closed cooled, had 1800 hours on it, and had the engine oil cooler.
I stick to 20/50 now.
For your 4.3, I can't think of any reason in the universe why you shouldn't use the T6.
Quote:
Metals (ppm)
Iron (Fe) 41
Chromium (Cr) 1
Lead (Pb) 23
Copper (Cu) 33
Tin (Sn) 1
Aluminium (Al) 4
Nickel (Ni) <1
Silver (Ag) <1
Titanium (Ti) <1
Vanadium (V) <1
Contaminants (ppm)
Silicon (Si) 11
Sodium (Na) 41
Potassium (K) 19
Additives (ppm)
Magnesium (Mg) 1043
Calcium (Ca) 1080
Barium (Ba) <1
Phosphorus (P) 1062
Zinc (Zn) 1340
Molybdenum (Mo) 65
Boron (B) 38
Contaminants
Water (%) 0.15
Coolant No
Physical Tests
Viscosity (cSt 100C) N/A
Physical / Chemical
Base Number (mgKOH/g) 6.2
5W40 is likely to shear a bit. So what. If it shears you still have a strong 30 weight in the sump. Plus, I bet your changing oil at least annually and not putting a ton of hours on it, so it will be an excellent choice. I believe the new CK4 spec calls for a more shear stable 5W40.
I'd use synthetic blend 15W40 Rotella or Delo in it myself.
Personally I would prefer a conventional 15/40 CJ4 HEDO or if forced into a CK oil I would choose one without an SN rating from what I understand right now.
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Prolly good for a 250 hour run on the meter, and I'll bet you change before that ... Many folks change at 100 hours for peace of mind.
I have been running T6 in marine applications (both gas and diesel) for a long time. It always performs well. Have never had an oil related failure.
As others have said in your case it'll shear down to a strong 30+ - just about where you want to be 
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