from 10w40 to 5w50

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Hello guys/girl I have an important question that I need to have answered I own a 13y old Ducati ST4,for the unfamiliars, this is a 2 cylinder 4t 'L' shaped V-twin of 916 cc (large bore, small stroke i think) I only have it for about 6 months now, so only drove it during winter time. At the end of the winter I gave it a large service during which I off course changed the oil. after consideration I used 5w50 instead of the prescribed 10w40. at first thing were oke, but after a while i noticed it starting to have problems with maintaining low rpm's and would sometimes stall at trafic lights at arrival. this slowly increased over time in frequency and severity. reasonably suddenly it stopped maintaining stationairy rpm (1250) at all.. and started to smoke.. blue.. and really fouwl irritating (eyes and lungs) smell it runs well at rpm's higher than 1500..though. anyway.. I feel like the 5w50 is to thin.. so it doesn't seal enough and therefore burns oil, leaks gasoline in the oil (thinning it further) and looses compression.. now my question is.. and this is the important part for me.. !!! Can this happen only by the difference of 10w40 and 5w50 !!! of is there an other cause!!! (or did this lead to another cause???)
 
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I looked at the spec sheets of Syntec 5W50 vs M1 15W50. The Syntec is thicker when its hot,and the M1 is thinner when its hot (M1 being 18 and Syntec being 20).
 
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An other cause could be (if the oil in the sump is also used to lube the gears) that the oil molecules sheared when abused by being squeezed by the gears. There is a reason oils are specified for motorcycle use. A good motorcycle oil has to be able to handle the abuse of the heat of the engine, the squeezing of the gears, and still be slippery enough for the engine, and not too slippery for a wet clutch. When you go with an oil with a wide viscosity range (such as 5W-50) in most cases you also have an oil with molecules that will shear easier than an oil with a narrow viscosity range (such as 10W-40), and even selecting a 10W-40 you should still stay with something sold for motorcycle use.
 
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Mono weight oils such as a straight 30 or 40 are less likely to shear than a multi-viscosity oil, if you can live with the thickness of the oil on start up (read only used in warm days). So if you must chose an oil that is not rated for motorcycles, stay with a mono weight if the temperature is always going to be warm, and a low spread from the first number to the second number if the the temperature is going to get cold. And you might want to change the oil more often if you are using the multi-viscosity.
 

Spikey

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Thank you all, that are some interesting responses... !! how likely is it that my problems will be solved when I change back to full synthetic 10w40 motor specific (dry clutch, should've mention that) greatz matthieu
 
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In my experience, the lighter the oil in an older carb'ed engine the more stable the engine idle since light oils have less viscosity change with oil temperature. That said I don't think the symptoms you've described is oil related. In my experience with motor bikes is that you'll know when the oil gets too "thin" since the gear shift quality will deteriorate. You haven't mentioned the brand of 5W-50 oil but many are shear prone GP III based oils so it is entirely possibly that the 5W-50 became lighter in service than a more shear stable 10W-40. When I was a kid and into bikes years ago, one house brand 5W-40 PCMO I tried in my first bike sheared so quickly, that after 500 miles when the engine was hot I literally couldn't change gears. Switched back to a 10W-30 which was much better but not as good as a straight 30wt oil. That dramatic difference motor oil can make in how a vehicle performs is how I got hooked on the stuff.
 
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