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Dear all, today I started with the restoration of my engine from my 305 dream. After this restoration I want to use it more often. Does anyone know if it is allowed to use multigrade oil ? Now i used the monograde SAE 30 but this oil is difficult to find and quit expensive. I thinking about to use 10W40, is that possible? Many thank for your help and advise. Regards, Eriv
 

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Dear all, today I started with the restoration of my engine from my 305 dream. After this restoration I want to use it more often. Does anyone know if it is allowed to use multigrade oil ? Now i used the monograde SAE 30 but this oil is difficult to find and quit expensive. I thinking about to use 10W40, is that possible? Many thank for your help and advise. Regards, Eriv
It's worth noting that the engine has the oil recommendation moulded into the upper crankcase cover, SAE 30 for temperatures above 15℃ and SAE 20 if it's cooler. (SAE 20W is a winter grade and has too low a viscosity given the high operating temperatures of air cooled engines.)

I have seen too many flame wars over the topic of which grade to use, with a fundamentalist camp insisting that only SAE 30 be used because that's what Moses read on a tablet (that looks a lot like a crankcase cover) that he received from God on a mountain. This ignores the fact that multigrade oils were just starting to be used when these bikes were designed. The recommendations for later machines from the late 60s and 70s are for multigrade, usually 10W30. Unfortunately, the SAE 30 that is available now is intended for lawn mowers etc. and meets the lowest API standard for motor oil, SA. See the Wikipedia article for more on this.

I'm in agreement with Bill H that 10W40 would be a suitable although I would add that it should be a motorcycle oil that is friendly to the wet clutch.
 

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Modern car oils, especially synthetics have friction modifiers that work well in most engines....unless the oils come in contact with a wet motorcycle type clutch. These oils then coat the clutch plates with slippery stuff that ruins the clutch. All oils designated for motorcycle use do not have these additives and so are safe to use with a wet clutch.
 

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Thank you Bill! I think it also but was looking for the confirmation 👍
Hello, 10w40 oil did not exist when 305 engine was produced. Straight weight oil was the norm. Those days the bikes were reluctant to idle when cold due to thicker oil in crankcase.
So the crankcase casting would be labeled for 30w as a compromise. 40wt oil would resist idle at cold engine start up, too thick, and 20wt too light on a hot day sometimes resulting in visible exhaust smoke at a traffic stop light. After multi grade oils became available they were recommended by Honda, 10w-40w being the most popular choice for a street bike. I would not use 20w-50 except in extreme (hot) conditions, or off road, hot weather, larger displacement air cooled models. 10w-40 MOTORCYCLE oil is my choice here in California. Good Luck.
HondaJohn.
 

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Modern car oils, especially synthetics have friction modifiers that work well in most engines....unless the oils come in contact with a wet motorcycle type clutch. These oils then coat the clutch plates with slippery stuff that ruins the clutch. All oils designated for motorcycle use do not have these additives and so are safe to use with a wet clutch.
Hello, 10w40 oil did not exist when 305 engine was produced. Straight weight oil was the norm. Those days the bikes were reluctant to idle when cold due to thicker oil in crankcase.
So the crankcase casting would be labeled for 30w as a compromise. 40wt oil would resist idle at cold engine start up, too thick, and 20wt too light on a hot day sometimes resulting in visible exhaust smoke at a traffic stop light. After multi grade oils became available they were recommended by Honda, 10w-40w being the most popular choice for a street bike. I would not use 20w-50 except in extreme (hot) conditions, or off road, hot weather, larger displacement air cooled models. 10w-40 MOTORCYCLE oil is my choice here in California. Good Luck.
HondaJohn.
Just a comment on nomenclature. The recommendation is SAE 30 above 15℃ and SAE 20 below that. Note that there are no Ws after the numbers. For oils not meant for winter have their viscosities measured at 100℃, close to operating temperature. For winter oils the viscosity is measured at 0℃ and a W is attached to the 'weight'. So, both grades that are recommended for these bikes are summer grades.

Multigrade oils are measured at both 0℃ and 100℃ because they combine the characteristics of both summer and winter oils -- they are thin when cold so that the engine is lubricated when started and until it warms up a bit. However, the oil does not thin out as much as much as a simple winter grade would and so behaves like a winter oil when cold and a summer oil when hot. That is why they are labelled something like SAE 10W40. The W does not stand for 'weight'. It stands for 'winter'. Using SAE 20W in one of these engines would probably be disastrous.
 

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Here's one more thing. Multigrade oils were available in the 60s but they were new on the scene. Just so that you know that the recommendations moulded into the crankcase cover are not definitive, I have attached a couple of pages from a CB77 owners' manual. Sorry for the quality.

Font Material property Parallel Rectangle Number


Note that 10W30 is recommended for "General purpose except for extreme hot and cold climates" which describes where most of us live.
 

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I'll give my two bits on this discussion (years later!) :-\ My Superhawk loves 15w40 diesel oil (Delo 400). I've never had a problem with clutch slip. I just tore the motor apart because of a problem with the head gasket (still original since 1966 and seems to have just come apart from fatigue). The motor is still in pristine condition, still hone marks on the cylinder (40,000 miles), cylinders are not oval in the least, nor hardly any wear on the pistons. A good 30 weight oil has marvelous specs, but as some noted, hard to get a good, detergent 30w. I live in a moderate climate, and the 15w40 works fantastic for my bike. Just my two cents!
 
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