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General Category >> Rubber Side Down! >> Mixing different viscosity oils.
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Message started by skrapiron on 07/11/08 at 21:40:58

Title: Mixing different viscosity oils.
Post by skrapiron on 07/11/08 at 21:40:58

I need the advice of you engineering types. KwakNut, if you're out there, I'd really appreciate you weighing in...

I'm fast approaching another maintenance interval on my bike. I took stock of what I have in the garage and found that at some point, I bought a bottle of Mobil1 syntethic 10w40 instead of the Mobil1 synthetic 20w50.

Will I cause any kind of compatibility problems by mixing the 2 different grade oils?

I used to add motor-honey to my Ford pickup every oil change to keep the oil consumption down. (it was a ford. the thing ate oil!) I know that motor-honey (STP oil treatment etc) is a viscosity modifier that prevents burn-off. I still had the same base oil, but with an additional additive package. No problem there.

My concern is with mixing the 2 different grade oils. Even though they are from the same manufacturer and should share the same PAO base oil, is there any potential for incompatibility between the 2 different grade oils? What about their additive packages, detergents etc? Will there be the potential for sludging, foaming or oil break-down if I blend the 2 together? I'm probably being paranoid, but at $8.00 a quart, I want to be sure before running out and getting another bottle of 20w50 and always having the orphan bottle of 10w40 hanging around......

Title: Re: Mixing different viscosity oils.
Post by BurnPgh on 07/12/08 at 01:59:04

I highly doubt you'll have an issue usually, however I remember getting the 20-50 specifically because the 20-50 had no "oil conserving" label where the the 10-40 did. That MIGHT cause an issue, but as I've only ever run super cheapo generic oil (changed every 1k mi) I can't say. If you've run the 10-40 in your bike with no issues I'd say your good. You'll only dilute what friction modifiers are in the 10-40 by adding the 20-50 aswell as thickening it up a wee bit in general... Just my opinion, I could be totally wrong.

Title: Re: Mixing different viscosity oils.
Post by craigq on 07/12/08 at 03:54:17

The Mobil 1's you are talking about their motorcycle oils, MX4T/Racing 4T 10W40 and V-Twin 20W50 right ? If so I wouldn't worry about it at all...

Title: Re: Mixing different viscosity oils.
Post by skrapiron on 07/12/08 at 10:21:02

I figured as much. Yes, they are both motorcycle formula oils.

I was concerned that the heavier oil might have different detergents or viscosity modifers that would be incompatible with the lighter oil.

I did some checking around on my own, and yes, I can mix them. I will essentially create a multi-weight oil with viscosity ranges somewhere between 10w and 20w and 40w and 50w respectively. The final viscosity does not corrolate geometrically with the difference between the 2 weights, but for arguments sake, I will end up with something close to a 15w45. Heavier than stock, but not as thick as the 20w50 that I have been running.

Title: Re: Mixing different viscosity oils.
Post by Steve M on 07/12/08 at 11:47:50

I'm pretty sure that as long as oil is motorcycle specification it is ok to put in.
Motorcycle oil has additives that are not in all car oils to cope with the stresses of having a multi-plate wet cluctch in the engine.

the first number is the winter viscosity and the second the hot (running) viscosity (hence 15w 40).
If you are in a climate where some of the year you are riding in very cold conditions then a 5w ?? or 10w ?? oil is best as it will get round the engine better on cold starts and minimise engine wear.

However it also applies that if you are riding in an area where the temp is very warm / hot a ??w 50 oil is better as that will keep it's viscosity better at high temperatures and help minimise engine wear.
I think this is particularly important in air cooled engines like the savage.

One of my bikes is a moto guzzi 100 sport V twin. This has huge air cooled cylinders and doesn't like being in traffic jams in the heat of summer!
The recommended oil for the guzzi 1100 sport is a 5w - 50 semi synthetic for year round use.
I am using 5w 50 semi synthetic in the savage as well now and hope to get a high mileage from the engine.

sorry if I'm spouting stuff you already know!
I hope some people will find the info useful?

Title: Re: Mixing different viscosity oils.
Post by barry68v10 on 07/12/08 at 17:30:32

3F273E2D3C253E23224C0 wrote:

The base stock plus the viscosity improver will yield the final weight...i.e. mix 10-40 and 20-50 and you get basically 15-45. The viscosity improver will combine oil molecules together from the base stock (the first number) to make bigger oil molecules/thicker oil. Some 20 weight may combine to 80 weight in the above example, but your overall mixture will flow like a 15-45.

I wouldn't recommend mixing oil more than 1 or 2 grades different, i.e. don't mix 5-20 with 20-50, and certainly don't mix 0-50 with straight 30! You'd end up with some ridiculously huge molecules floating around in your engine! Mixing 5 with 10 or 10 with 15 or 20 is okay (first number.)

Title: Re: Mixing different viscosity oils.
Post by willrph on 07/13/08 at 05:38:41

Wow, this was agreat thread. I need an oil change and wanted to switch to 20W50. My musings over not using some left over 10W40 are no longer a concern. With the price of synthetic oil, just don;t want to waste a drop.

Title: Re: Mixing different viscosity oils.
Post by gazoo07 on 07/13/08 at 18:28:14

There will not be any problem with mixing the oils as long as they are both motorcycle oils and contain no friction modifiers. at the very worst you would only be lowering the over all viscosity slightly and since the Savage already calls for 10W40 it will not be an issue. DMC
www.lubedealer.com/1lube

Title: Re: Mixing different viscosity oils.
Post by justin_o_guy2 on 07/14/08 at 06:11:53

Barry, I would never have considered the oil actually bonding, molecule to molecule, creating longer polymeric chains. It is polymeric, right?
So, if I have some oil thats been chewed up, sheered thru gears & cycles thru crank & bearing holes & losing some of its lubricating ability, I add some fresh oil of different viscosity & it will bond with the oil, creating longer molecules?
I am not suggesting that we not change oil, just trying to grasp the concept. Will shorter, broken off, bits of a molecule grab onto a new one?