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I ride a 2015 Yamaha FZ-07 (around San Francisco / Bay Area), which is water-cooled and supports 10w30, 10w40, 10w50, and 20w50 motorcycle oils.

However, the manual says not use 10w30 oil when the ambient temperature exceeds 70F/21C (and recommends 10w40 or heavier instead).

Why are lighter oils contraindicated for hotter ambient temperatures?

If I use a full synthetic oil (which is probably going to be hardier), is it still bad to run a lighter oil when it's hotter outside?

The engine temperature itself is much hotter anyway, and stays pretty much the same (180F/82C) no matter what the ambient temperature is.

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    Is there a reason you are considering going against the manufacturer recommendation? Are you sitting on a stockpile of the wrong oil or something? – cory Nov 30 '16 at 16:19
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If the oil is too 'light', then it will be too thin when hot. Ambient temperature absolutely affects engine temperature, or at least it can. That is counteracted to a large degree in a modern, water-cooled engine, but it's probably there out of caution, standards from an earlier time, and general best-practice.

If it says above 70F don't use the light oils, that means that that oil won't be suitable in non-cold temperatures--if you're almost always in the highest range of heat your engine is rated for, that's too thin to be ideal, and you'll see faster wear on the engine.

A 'full synthetic' oil will just have fewer impurities, by the way, but that doesn't affect how you should interpret its rating. Viscosity and temperature is consistent with rating, fully synth or otherwise.

tl;dr: follow Yamaha's instructions to get best life of engine. You'll probably sell it before you ever see the effects of whatever oil choice you make, of course... But it's good karma to take care of an engine that new. ;)

  • So essentially, too light oil at higher temperatures might result in bad piston/oil rings? (I imagine the clutch/transmission impact might be less? Also, if it's just because the manufacturer is worried about lighter oils breaking down faster at higher temperatures, wouldn't that be mitigated by going full synthetic?) – ManRow Dec 12 '16 at 23:07
  • @ManRow All points of friction suffer with oil that's too light. This would be bearings, cylinders, clutch, etc.. In cars, transmissions have a separate oil comparment, but in most motorcycles, the transmission uses the same oil (one of the main--but not only--reasons oil breaks down faster in motorcycles). Full synthetic will not break down slower--it's just free of contaminants. The problem isn't breakdown--It's viscosity. A thinner oil in a higher temp just won't be viscous enough, synth or not. Heat thins oil. So the hotter your operating temperature, the thicker the oil you want. – Kyle Baker Dec 12 '16 at 23:21
  • To make it clear: you are probably not getting your engine hot enough to damage your oil. But if your engine gets too hot for the oil viscosity you have, the oil will get thin while the temp is high. The oil is fine, though. It'll go back to being thick when it gets cold again. However, the engine won't have sufficient lubrication if the oil is too thin, causing excessive friction and damage. Engine oils use specially designed molecules to be thinner at low temps and thicker at higher temps, to counteract nature and provide a consistent viscosity at different temps, but have limited range. – Kyle Baker Dec 12 '16 at 23:26

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