5w40 shearing????

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Oil guru's.....will a "synthetic" 5w40 HDEO shear more than a dino 15w40??? Some people tell me that a "marine" engine that runs constant high rpm's NEED'S a
15w40 oil to prevent shearing..but I thought that synthetic oils dont shear like dino oils do?? Also some old wise mechanics I speak with tell me that when "cold" 15w40 oils protect better because they provide a "cushion" upon startup (like honey on the moving parts) unlike a 5w40 that will "drain" off of engine parts like water. Please help me?? I have this BITOG addiction : , )
 
Quick reply.

I don't thing that oil "drains" off surfaces like water. Pulled apart a few engines, including a blown short block that had no heads or sumps, and had been standing end on for a long time...got very oily, around the big ends/crank journals.

As to the 5W versus 15W...depends again on the "synthetic" definition used.

15W-40s (and 20W-50s) don't have a significantly increased Viscosity Index over some of the available straight weights, and don't in general have heaps of VI improvers...part of the reason that they were widely accepted.

A 5W-40 synthetic should be pretty stable depending on the definition adopted.
 
A Group III based 5w-40 contains a fair amount of polymeric thickener and will shear significantly in a boat motor. I'd recommend a 15w-40 or 20w-50, PAO/Ester based synthetic that uses little or no VI modifier. I'd go with the latter if you tend to see much fuel dilution in your application.

Best way to theoretically determine shear stability in actual service is to take the Vis @ 100C and divide it by the HT-HS viscosity @ 150C. The smaller that number, the more shear stable the oil is. For a Marine application, I'd want something with
Vis @ 100C/HT-HS @ 150C to be < 3.5
 
A 15W-40 Group II conventional should be at least as shear stable as a synthetic 5W-40 -- if not a little better generally, all things being equal.

A 15W-40 conventional (Group II) starts with ~ 7.0 cSt base oils, then adds VII (viscosity index improvers) to get to 40 wt range -- say 15.0 cSt.

A synthetic 5W-40 starts with 6.0 - 7.0 cSt base oils (6.0 = Group III, 7.0 = Group IV) and adds VII's to get to 40 wt range. It's the much lower cold specifications of the 5W compared to 15W that causes one to have to use thinner base oils for the synthetic 5W-40 compared to the conventional 15W-40.

Most synthetic 5W-40's are probably Group III, or contain majority Group III (at least here in North America) with some Group IV and will require more VII's to reach the 15.0 cSt 40 wt range than a conventional 15W-40. If you start out at 6.0 cSt, the synthetic will have to bridge a little larger viscosity gap to get to 15.0 cSt than does the conventional which starts out at 7.0 cSt, and this requires more VII's. Since the synthetic 5W-40 is relying more on VII's to get to 15.0 cSt, and it is the VII's that shear down and cause the oil to lose viscosity -- not the base oils -- it will make the synthetic 5W-40 not quite as shear stable as a 15W-40 conventional.

At best, if the synthetic 5W-40 uses the more expensive PAO (Group IV) base oils, it could be a match for the shear stability of the conventional 15W-40. But, I say all things being equal because there are different VII's and some are better than others, and a synthetic may use slightly better VII's.

On paper, I think the 15W-40's conventionals should have the edge in shear stability generally compared to synthetic 5W-40's.
 
just put Mobil 1 5w40 in it and for get about it.
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it will last many years with Mobil 1 in it and i have been using Mobil 1 since 1985 with no problems.
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Marine engines have very high load at mid-range and higher rpm. Boats have lots of resistance and yet they tend to run cooler due to coolant intake.

The thinner oil will shear sooner than the thicker oil. The lower operating temps typical of a marine engine may not let the VII get to optimum. That would favor the 15w-40.
 
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