Oil weight vs power output. (Small test) on 25cc 4 Cycle Honda

Feb 19, 2009
The Woods of NY
This is a strictly non-scientific just observation with the results I experienced using all Amsoil Synthetic 5w30, 10w30, and a diesel grade 15w40 oil.
The engine is a small Honda 25cc 4 cycle on a Weedwacker. It has valves and a timing belt 😂 I made sure the carburetor was adjusted correctly and cleaned, along with clean air filter, fuel filter, sparkplug, and valves to correct cold spec. I am using e10 87 octane with a touch of Lucas upper cyl lubricant and carburetor cleaner. The oil capacity is around 3 ounces and factory spec is 10w40. The oil was changed warm and used the same amount of oil on every oil change when switching grades.


5w30, and 10w30 no perceivable difference in power output resistance in pull string or “smoothness”’of engine. No smoke, abnormal sounds and the engine did not blow. If I had a blind test I would not be able to tell you the difference between the two.

Now the diesel grade 15w40 amsoil is a completely different animal. This small little motor really felt the effects of this “thick” engine oil. It was slower to rev, and no exaggeration had half of the engine output power. It even smoked a little.
The engine did not like this grade of oil.

So thinner engine oil does give you “more” power or I should say least resistance at expense of wear maybe? I’m not sure.

To note: I use Amsoil 5w30 and change it every 8-10 hours of use. The oil still comes out honey colored and is not littered with metal flakes, also it does not burn any oil in the 5w30 flavor. I think even the factory recommended 10W-40 would be slightly too thick for this little motor. I wish I had a 10W-40 to test but I didn’t.

Again this is strictly just an observation of a couple different grades of oil I had sitting around in a small four cycle engine.
I would think though the effects would be the same thing on say even a regular gas engine… if I were to put a 15w40 diesel grade engine oil in the kia, it probably would feel very lethargic and would have no true benefit.. in a engine spec for a 5w30.

I don’t know not trying to start a war here just a casual observation.

On a small engine on initial startup, I can see your point. I think it's all irrelevent after the engine has run a couple minutes and warming up the oil . Don't like that internal timing belt that Honda likes to use..
The timing belts are not an issue, and have been known to hold up to pretty severe abuse. I've actually never seen a broken one.

As far as your testing, I would say it is inconclusive, especially since you are running fuel that also has Lucas injector cleaner and carb cleaner in it. I wouldn't recommend doing that. I've run 15w40 in small engines that recommended 5w30, I only noticed a slight difference in extremely cold temperatures (-5) when trying to pull them over. To me the issues you mentioned almost sound related to being slightly overfilled, not a change in viscosity.