Severe Service OCI vs Severe Service Viscosity

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192
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Northern Virginia
My 2008 Subaru Impreza owners manual recommends a 5w-30 SM oil. The maintenance schedule recommends a 7500 mile OCI for normal or 3750 mile OCI for severe service. Severe service might be defined as short trips, extended idleing, towing, use in high heat, or other heavy duty applications. For Subaru, they recommend, for severe service, API SM in 30, 40, 10w-50, 20W-40, or 20W-50. Probably other manufacturers have similar allowances for different oils. If one were following a severe service OCI, would it not also be logical to use the severe service oils recommended? I'm not tracking why there would not be a correlation between the usage, change interval and oil weight. In other words, if Subaru felt your usage warranted a severe service OCI why they would not openly say you also need to use a heavier weight oil. Or, vice versa, if you follow a 7500 mile OCI then you should only use 5w-30. I got the part about oil changes are good for business, but is there a valid maintenance reason severe service schedules and severe service oils don't go together - at least as described in owners manuals.
 
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1,084
Location
Florida, USA
The reason they recommend those viscosities for severe service is because conventional oil is prove to viscosity breakdown. Just use a 5W-30 Full Synthetic and you will get the best of both worlds (good fuel mileage and superior engine protection against viscosity breakdown). You should be able to go 7000 miles even with moderately severe service with a full synthetic.
 
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ontario canada
because heavier does not equal better. i have severe service because of short trips and cold weather, so i use LIGHTER - 5w30 synthetic mixed with 5w20 conventional. if it was high heat or towing i might use a synthetic 10w30, which would last 7500 miles. it's soon time for me to switch over to 5w30 because it's turning to summer in another month.
 
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Pottstown, PA
Scoobie said it well.
 Quote:
If one were following a severe service OCI, would it not also be logical to use the severe service oils recommended?
Does your severity challenge the regular viscosity? If so, how? Most Chevy pickups spec a 5w-30. The only thing that changes is the length of service, not the viscosity.
 Quote:
I'm not tracking why there would not be a correlation between the usage, change interval and oil weight.
You may be in severe service because you're consuming radically more fuel for standard service than your odometer can integrate for an appropriate OCI. Suppose 90% of your driving is bumper to bumper urban driving? You may get 60% of your normal fuel economy and run your engine many hours with little mileage. The 3750 covers that sorta of mismatch. Suppose you are towing, but are also in Frostbite Falls, ND with -40 starts? I doubt that 20w-50 would be your oil of choice. Subaru was at least kind enough to give you a wide option of viscs to use. You just have to apply some dusty shelf reasoning to figure out which one. We're a land of one size fits all ..and have been for a long time.
 
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I believe that more people fall under severe service main't than normal service. Reading through various OM's its not that hard to fall into severe service, especially if you live in a metropolitan area or have kids in school and use the car like a soccer mom. AD
 
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 Originally Posted By: ericthepig
A bit of Jerry Seinfield here - but what's up with that number - 3750? Just a couple of hundred from 4k. That's just weird.
It is one half of 7500, the normal OCI. Everything has to be divisible by 15,000 so they can fit it on the maintenance chart with all the other stuff like radiator and transmission fluid changes, etc.
 
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Hey WagonBoss, I've thought the same thing. My 08 Forester manual is the first manual where I've seen such a selection of oil viscosities. I do what Mark888 stated, use a good 5w30 or 0w30 syn under all conditions.
 
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 Originally Posted By: PT1
Just use some PP 5w30 and go 5000 and all will be fine with it.
Or Synpower or M1 or RP or....
 
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Canada City, Canada
 Originally Posted By: ericthepig
A bit of Jerry Seinfield here - but what's up with that number - 3750? Just a couple of hundred from 4k. That's just weird.
In the rest of the civilized world, there's a crazy new measurement standard called the "Metric" system. One might note that it's exactly 6000km's.
 
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Buffalo, NY
 Originally Posted By: Mark888
 Originally Posted By: ericthepig
A bit of Jerry Seinfield here - but what's up with that number - 3750? Just a couple of hundred from 4k. That's just weird.
It is one half of 7500, the normal OCI. Everything has to be divisible by 15,000 so they can fit it on the maintenance chart with all the other stuff like radiator and transmission fluid changes, etc.
Yes. Several manufacturers once used it, mostly Japanese but I think VW did as well. I also believe they did it to allow most people to get to three months before changing their oil under a "severe" standard...
 
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 Originally Posted By: D189379
In the rest of the civilized world, there's a crazy new measurement standard called the "Metric" system. One might note that it's exactly 6000km's.
The metric system has nothing to do with it. Anyway, the metric system is completely arbitrary, and based on the coincidence that humans have ten fingers.
 
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39,806
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Pottstown, PA
 Originally Posted By: Nickdfresh
 Originally Posted By: Mark888
 Originally Posted By: ericthepig
A bit of Jerry Seinfield here - but what's up with that number - 3750? Just a couple of hundred from 4k. That's just weird.
It is one half of 7500, the normal OCI. Everything has to be divisible by 15,000 so they can fit it on the maintenance chart with all the other stuff like radiator and transmission fluid changes, etc.
Yes. Several manufacturers once used it, mostly Japanese but I think VW did as well. I also believe they did it to allow most people to get to three months before changing their oil under a "severe" standard...
Well, the intervals are 3months or 6months with mileage limitations and a "whichever comes first" hierarchy. My 6month schedule is 7500. My 3month schedule is 3000. Without some designated severe condition you can be in a 6m/6k scenario ..or 5k/6months.
 

WagonBoss

Thread starter
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192
Location
Northern Virginia
Thanks for the recommendations, but I was looking for discussion on the logic of lubrication and just using the owner's manual as a start point. What I take away here is that manufacturers' have used the OCI as their variable in lubrication moreso than the type/viscosity of oil. I suppose this is easier to sell to consumers than saying you have to change oil viscosities based on your driving. That could get a little complex. Seems though, in the example, there is room to tune your lubrication somewhat. Any value? Probably not, other than a thinking exercise.
 

MolaKule

Staff member
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21,505
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Iowegia - USA
 Quote:
What I take away here is that manufacturers' have used the OCI as their variable in lubrication moreso than the type/viscosity of oil. I suppose this is easier to sell to consumers than saying you have to change oil viscosities based on your driving. That could get a little complex. Seems though, in the example, there is room to tune your lubrication somewhat.
The viscosity, in this case the recommended 5W30, gives you lubrication for both extremes of temps and economy. The Oil Change Interval is an average based on the manufacturers testing. Yep, tuning up your lubrication is what we is all about here.
 
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6,388
Location
Washington St.
Use a viscosity that suits the temperature of the oil in service. If you get that engine extremely hot, use a heavier viscosity oil. If not, use 5W-30. What's hot?...how about max loads & high speeds in very hot summer weather. Note that humidity doesn't count, just temperature. Heavy hauling in Phoenix in summer...higher viscosity. Synthetic oil is great when either very hot or very cold. Highly recommended. I'd use a top quality synthetic and run the regular oil drain intervals, or maybe use a gasoline engine 5W-40 (Conoco/Phillips/76/Kendall is one good one; there are others) if running heavy & hot.
 
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