20W-50 to 10W-40? Is it safe?

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Phatboyaa.131

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Mar 26, 2025
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Hello! I've just got here from Reddit and the amount of resource overwhelms me. I need advice on choosing oil. My bike is a Supercub C100 series (Locally called Astrea Legenda) released on 2002. It's been rebuilt recently, changing the piston kit, rehoning the cylinder, reseat the valve, new valve seal, etc and I want the best possible oil for it. Manual book says to use 20W-50 oil, but absurdly it's more expensive here than 10W-40 while having lower specs. So far the highest spec for 20-50W is mineral based or semi sinthetic, API SL while 10W-40 get full sinthetic and latest API with almost similar price. I really inclined to use synthetic based oil with most recent API rating, can I use SAE 10W-40 for my bike? For the record ambient temperature here is around 25 C to 30 C, maybe up to 40 in traffic. Thanks in advance!
 
I don't think I've really seen anyone go from a thicker to a thinner oil, normally see the other way around. I have no clue if it will or won't be safe, but its always a good bet to stick with user manual, especially in applications like this. Others will have a ton more information than me.

Do you have a Walmart near you? They now offer a 20W50 motorcycle oil - may be location dependent, however.
 
MurkyAsparagus said:
I don't think I've really seen anyone go from a thicker to a thinner oil, normally see the other way around. I have no clue if it will or won't be safe, but its always a good bet to stick with user manual, especially in applications like this. Others will have a ton more information than me.

Do you have a Walmart near you? They now offer a 20W50 motorcycle oil - may be location dependent, however.
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I'm in asia so unfortunately no. I'd want to stick to manual, but with the weird pricing here I'm just wondering because it seems like a joke that thicker oil with inferior spec priced the same as the higher spec thinner oil. If sticking to 20w-50 is the best choice I'd stick with it then, hope others can chime in.
 
In the past, Honda has spec'ed 10w-40 in similar bikes. It certainly won't immediately blow up if you use the lighter grade, but you live in a hot climate and I think that's an air cooled bike so I would be mindful in hot weather.

I bet it only takes about a litter per change so is it really worth it to switch?
 
Phatboyaa.131 said:
I'll try using it for my next change. What should I watch for during usage to know if it's suitable or not for my bike?
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Most of the time it's valvetrain or rotating assembly noise but I doubt a 40 grade will cause that. Now if you were wanting to go from 20w-50 to 10w-30 I would say no because that is too thin.
 
ripcord said:
In the past, Honda has spec'ed 10w-40 in similar bikes. It certainly won't immediately blow up if you use the lighter grade, but you live in a hot climate and I think that's an air cooled bike so I would be mindful in hot weather.

I bet it only takes about a litter per change so is it really worth it to switch?
Click to expand...
I mean if it's more benefit than harm using similarly priced oil, I'd be down but I'm not too sure
 
JavierH19 said:
Most of the time it's valvetrain or rotating assembly noise but I doubt a 40 grade will cause that. Now if you were wanting to go from 20w-50 to 10w-30 I would say no because that is too thin.
Click to expand...
Alright note taken. I've finished reading the motor oil 101 on this web, most of it goes over my head but I notice Bob himself use a degree thinner oil in their car
 
For an air cooled engine I will say 20-50 , for water cooled one 10-40 or 10-30

I used to run an XR650L specs engine on a motard bike from Honda FMX650

This old days run 20-50 and for 70.000Khm almost don't see any problems, I sell the bike to someone else.

Even 20-50 on that engine becomes water and I add a small oil radiator from XR400 and be a little better.

Today Honda on manual suggest 10W30
I assume they have brain damage (my opinion) because the 10-40 and become water.

So I will say on a cub engine , 20-50
 
I am a fan of thinner oil, however with an air cooled engine I wouldn’t recommend it. I used Mobil 1 15W50 in my Harley twin cam 103 for over 60,000 miles and good results if you can get it. It’s a synthetic and reasonably priced
 
10/40 NO
20/50 YES

The 10/40 is an inferior spec simply because your bike requires a 20/50

By the way, an oil labeled SL or lower, will have more phosphorus and zinc in it than a newer oil, such as a SN
 
What is the brand of 10w40 you can buy that is synthetic? Are there any used oil analysis on this forum that show how that oil performed?

I would go 20w50 semi-synthetic because there are less viscosity index improvers in that oil versus a 10w50 or a 15w50. However if you choose 10w-40, if Mobil 1 10w40 4T is available I would use that. Your ambient temperatures aren't extremely high, 40° C in traffic occasionally isn't going to stress oil.

In the overall picture, the bike you are talking about has been sold by the millions and millions around the world. Plenty of folks don't even have 10W-40 oil to put in it or change it regularly. I think you're going to be fine whichever way you choose
 
My 1979 BMW R45 airhead specs 20W50 but it runs the oil quite cool which is not surprising given it has almost the same cooling fin area as other models with double the HP. I started with 20W50 then 15w40, 10W40 and for 18 years it's run on a quality 5W40. The oil designation is not the only criteria, I consider the actually viscosity of the oil at the temperature it runs at and I'm more than happy that my 5W40 @ 85 Deg C is a similar viscosity to a 20W50 @ 100 Deg C. Bear in mind that due to lower losses, a thinner oil will run cooler than a thicker oil so the viscosity drop is not as big as you might otherwise expect. If you are going to run a thinner grade also look for as high a HTHS as available in that grade.

Ambient temperatures sound to be a good deal higher where you are so I'd measure the oil temperature after a long run before making a decision.
 
In a hot climate with an air cooled engine I would stay with the 20w50. My Yamaha air cooled twin speced a 20w40 when I bought mine but 2 years later they changed the recommendation to a 20w50. The 40 will get real thin when you get in traffic with lots of stop and go on a hot day. I even considered running a 60 but decided to just not drive in traffic on hot days.
 
Objectively speaking it is safe...

As we learn oils are only graded on flow in cSt (centi stokes) and
they are not graded on thickness in inches nor weight in ounces...

1 cSt is the flow of water...

12.50 to 16.29 cSt is the flow of 40 grade...

16.30 to 21.89 cSt is the flow of 50 grade...

2000 cSt is the flow of honey...

Given the same temp different grades flow at different rates powered by gravity...
The measurable difference between a 40 and 50 grade is this much flow powered by gravity
As we can see the lower the cST number the quicker the flow... quicker flow does not risk
more wear... quicker flow equals lower operating temps... quicker flow equals more HP...
flowtest40v50-jpg.238471
 
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