In my S4 i have been using Motul fully synthetic 10-40w oil however i have now been able to get Motul full synthetic 15-50w oil and it does make a difference the bike seems smoother and a bit mechanicily quieter ...i know its subjective but it what i feel especially in the Thai heat it can only help the temp here never falls less tha 70f mostly 90f
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I'm sure the 15W-50 will be fine. Both 15W and 20W-50 are well within Ducati recommendations for your climate.