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Hi everybody i am asking this question mostly in the context of motorcycle engine but i think the same basic principles apply on a car engine as well. I have changed my motorcycle engine oil with ~3lt of Ipone 10.4 semi synthetic oil. I have a spare bottle of Ipone 10.5 semi synthetic which is essentially the same oil (semi synthetic with exactly the same JASO MA certifications) but with a slightly thicker viscosity. Is it OK to mix a small quantity of this oil (about 200ml) into my engine? In general is it ok to mix variations of the same oil in an engine? What happens if this mix reaches 50/50 proportions? How is the viscosity of the final product regulated??? Thank you

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After a more careful search i found the following on a forum thread (https://www.600rr.net/vb/90-oil-lube/133221-mixing-oils-different-viscosity.html)

All mineral oils are produced from the same basic petroleum feedstocks. Different oil viscosities will not separate or react negatively to each other because the base oil is molecularly the same. It's the different oil additive systems between brands that shouldn't be mixed - if you're concerned about the oil performing exactly how it was designed. Oil additives include: Anti-oxidants to prevent thickening at high operating temperatures. Pour point depressants which lower the temperature of wax coagulation alkaline materials to neutralize acids formed during combustion. Rust and corrosion inhibitors. Detergents to reduce sludge and varnish. Dispersant additives to hold contaminants in suspension. Extreme pressure additives to prevent metal to metal contact under high loads. Viscosity index improvers in formulating muilt-grade (viscosity) oils.

The exact type and amount of these additives varies between brands and when mixed may not function as efficiently as desired. Different viscosities of the same brand oil will have different amounts of viscosity index improvers (polymers), but otherwise the additive systems will be the same.

From the Chevron.com site: "Can I mix different viscosity grades of motor oils?" "Yes. It is always advisable to not mix motor oil brands, however, different viscosity grades of the same brand motor oil are compatible. Be aware that mixing viscosity grades will turn out a product that is different in viscosity than either what was originally in the engine or what was added."

From Shell.ca: "If you mix viscosity grades such as a 5W30 low-viscosity oil and a 10W40 higher-viscosity oil, it is reasonable to expect that the resulting product will have viscosity characteristics which are thicker than the 5W30, but thinner than the 10W40. This change does not reflect incompatibility - it's simply a re-balancing of the viscosity characteristics. In all other ways, the product should work as expected. But there's absolutely no danger about incompatibility resulting from mixing engine oils," Miller says. "We've tested all of our grades and brands, and we haven't observed any problems."

From Mobil1.com: "For our customers to choose a viscosity grade, we recommend they follow the engine manufacturer's recommendations as indicated in their owner's manual. There is no need to mix two Mobil 1 viscosity grades when one will do; however, we see no problem mixing different SAE grades of Mobil 1 Tri-Synthetic series motor oils."

I agree that mixing oil viscosities isn't really necessary though. 5W-30 is going to have the same viscosity as 10W-30 when the oil is hot... might as well just use 5W-30 for its cold temperature "flowability". So the viscosity mixing argument is kind of pointless

  • I used to buy the Mobil 1 5W50 which used to be easily available, but now it seems to be harder to find, sorted summer / winter decisions nicely and, no, it definitely was not specified by the manufacturer... – Solar Mike Nov 15 '18 at 15:00
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yes you can mix same brand and type oils with different viscosity and it will be fine as long as you don't mix them 50/50.If you mix 200 ml of oil with the rest of oil needed for your engine you will be totally fine. However keep in mind that mixing viscosity of oils is not really recommended and it shouldn't be your usual practice.

  • I hope engines are designed to work with "some kind" of oil viscosity, they tend to be classed as Newtonian... – Solar Mike Nov 15 '18 at 13:38
  • @SolarMike did i say something wrong? – AsenM Nov 15 '18 at 13:43
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if they were meant to be mixed... believe someone would have the mix copywritten on the market. sadly it's not, so I'd say not a good idea.

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