Does 10w50 + 20w50 = 15w50?

[Bob_1978]

I buy a case of 6 quarts of Castrol 4T synthetic for my oil changes, 10w50 in the fall and 20w50 in the spring.  (I live in Seattle so can ride pretty much all year around.)

 

Since my 2013RT only takes 4 quarts, I now have 4 leftovers -- 2 quarts of 10w and 2 quarts of 20w

 

Since the original recommendation from BMW was for 15w50 (which Castrol no longer makes, or else I can't find it), any harm in mixing the two?

 

I know we HATE oil threads but I can't find an answer to this one.....

[dirtrider]

On 4/17/2022 at 10:16 AM, Bob_1978 said:

I buy a case of 6 quarts of Castrol 4T synthetic for my oil changes, 10w50 in the fall and 20w50 in the spring.  (I live in Seattle so can ride pretty much all year around.)

 

Since my 2013RT only takes 4 quarts, I now have 4 leftovers -- 2 quarts of 10w and 2 quarts of 20w

 

Since the original recommendation from BMW was for 15w50 (which Castrol no longer makes, or else I can't find it), any harm in mixing the two?

 

I know we HATE oil threads but I can't find an answer to this one.....

Afternoon Bob  

 

It will make something but unless you send it out & have it tested, (WAY/WAY/WAY) too expensive to do, then you won't won't know what you have. 

 

Have you looked into   Liqui-Moly Synth 4T 15W-50, there are a number of us BMW boxer riders that are now using Liqui-Moly Synth 4T oils. 

[Bob_1978]

Great advice - 4 lites of Liqui - Moly going for $43 on Amazon so less expensive than Castrol.  I'll cycle through my remaining Castrol then make the switch to a single, year-round oil.

[Hosstage]

I would have no problem mixing those two different weights together and running them.

[Skywagon]

I have a similar experience of half empty things.  I just mixed Makers Mark and Jack Daniels together.  I'm still running fine.  Never to waste those last couple of ounces.  I'm decades out of warranty.

 

OK seriously......Oil is too cheap to risk an expensive engine IMHO.  I suspect there is a 99.9999% chance you will be fine, but you have to buy oil eventually, why not buy it now while we still can.

[Hosstage]

So you're saying that there is a .0001% chance of a problem occuring by mixing two quality oils of the same type and brand with a difference in winter weight of 10 points? I'll take those odds every time.

Remember, the answer to every oil thread asking which is the best, is some.

[Rinkydink]

I’ve always subscribed to the “any oil is better than no oil” theory. Seems to have worked fine so far…

[John Ranalletta]

1 hour ago, Rinkydink said:

I’ve always subscribed to the “any oil is better than no oil” theory. Seems to have worked fine so far…

 

One of our past members, Ed, from Florida suggested when changing bike oil, the rider should sneak up from behind and hide the label on the oil container so the bike can't see it.

[dirtrider]

14 hours ago, Hosstage said:

So you're saying that there is a .0001% chance of a problem occuring by mixing two quality oils of the same type and brand with a difference in winter weight of 10 points? I'll take those odds every time.

Remember, the answer to every oil thread asking which is the best, is some.

Morning Hosstage

 

It might be the same oil type & same brand but it might not be the same chemical makeup between the oils to get the desired viscosity spread in each.  If the chemical makeup or blend is different then it is a crap shoot in what you end up with.  (there is a lot more to it than just adding 10+20 then dividing by 2)

 

Obviously the oil's are probably close enough to not have a catastrophic engine failure due to mixing but what about the other long term protection qualities? Are those effected by the oil additive makeup differences?    

 

Call the oil company's customer service number on the oil viscosity & type that you intend to mix,  THAT is the real answer, but we can all pretty well guess what that answer will be. 

 

 

 

 

[Hosstage]

I still think it will be fine, short and long term, very doubtful it will harm the motor in any way. 

[dirtrider]

1 hour ago, Hosstage said:

I still think it will be fine, short and long term, very doubtful it will harm the motor in any way. 

Afternoon Hosstage

 

I have been involved with enough unforeseen lubrication issues over the 50+ years that I have been in the automobile  engineering business to not make a recommendation  based on "I think". 

 

Common sense tells me that if 20w50 oil was OK with BMW then they would have included that in the riders manual as an optional oil viscosity.  Mixing 10w50 & 20w50 still leaves two different oil molecule size classes  (mixing it doesn't change that). One of the claimed traits of decent synthetic motor oil is similar sized & similar shaped oil molecules.  Would this mean that at very cold ambient temperature ride off that the 20w part  would impede oil flow through the oil filter element therefore forcing more unfiltered oil to flow through the filter by-pass valve for a longer period (I don't know this for a fact but it would be a worry of mine until I knew for sure).     

 

It probably wouldn't be a short term issue but if I had my choice the more oil I have flowing through the filter element rather than around it the happier I would be. 

 

 

 

[Hosstage]

Fair enough!

[terryofperry]

Widman has a vicosity calculator you can use if you like, pay attention to the note: (and as DR points out) "Note: You should only mix oils of the same brand and formulation"   

 

https://www.widman.biz/English/Calculators/Mixtures.html

 

Terry

 

[dirtrider]

7 hours ago, terryofperry said:

Widman has a vicosity calculator you can use if you like, pay attention to the note: (and as DR points out) "Note: You should only mix oils of the same brand and formulation"   

 

https://www.widman.biz/English/Calculators/Mixtures.html

 

Terry

 

Evening Terry

 

I have tried to use that calculator  before (for fork oil mixing to try to get a custom fork oil viscosity in my precision valved dirt bikes) but it is not exact as it uses the oil's temperature and  cST. It's almost impossible to find the cST ratings on fork oil.

 

The cST specs of motor oil is pretty well fixed for the 10w part (usually around 4.1 cST @ 0°c but that must vary a little in the real world, it just isn't a published rating)  but the 15wXX & 20wXX has a hi/low band that can effect the results as a 20wXX  can stack up very close to a 15w on the low end or on the high end be very close to a 25w. Again, the problem here is SAE only rates motor oil @ 0°c  and 100°c  so finding the cST of the oil (that you want to mix) at -5°c or 40°c to plug into the viscosity calculator is about impossible. 

 

If you wanted a 15wXX equivalent but mixed a 10wXX with a low cSt 20wXX (ei 1cST above a 15wXX) you would end up with an oil well below a 15wXX) 

 

I guess if you sent the both oils out to be tested for the  -5°c cST (probably the most important area for cold oil flow in a motorcycle) or at 40°c cST  then used those numbers it might be somewhat accurate as long as the oil chemistry is compatible.  

 

It seems that viscosity calculator is more written for single weight oils than multi-weight oils. 

 

Still probably best to just use the oil at it's designed viscosity then if you want something different just buy that viscosity oil as it is tested/certified for that viscosity at initial blending.  

 

  

[Hosstage]

Send the extras to me and I'll run them in my Harley. That big dumb tractor motor doesn't really care if the oil is a little mismatched...

[9Mary7]

this is why I love oil threads, people just can't resist......

[6speedTi]

Mobil One has a 15w/50. I've been using it on my 2012 RT after break in. That was a recommendation from the dealer. 93,000 miles and my oil consumption has not increased since break in. Maybe I am fortunate but it works for me. 

Ride safe, enjoy life. 

[lkchris]

50 is 50 is 50

 

The W means WINTER and is applicable only to cold starting and different W numbers do not change the 50

 

A decade or more ago I looked up Mobil 1 specifications and the cold pour points were identical for 0W, 5W, and 10W.  When your engine is warm, the only number in play is 50.

 

Given full synthetics, outside (ambient) temperature is of little concern and viscocity choices are more applicable to engine clearances.  50 is mostly applicable to air cooled engines which have larger clearances since they have greater expansions from heat.  Thermostatically controlled water cooled engines usually are fine with lower numbers.  Current Toyotas specify 0W-16.  Oil thickness is also a fuel consumption factor, but I think this is little concern with motorcycles.