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Hi, I have Honda CM400c 1981 , only 4K.
Which engine oil SE 10W-40 is correct: synthetic, semi-synthetic, or standard.
I have read that full synthetics can cause the engine's fluids to leak.
Is that so?
 

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Yes changing from conventional to synthetic can cause leaks. You can try it but since you've probably got original gaskets you might want to stick to conventional to avoid disassembling the top end. Just get a motorcycle specific oil (or the conventional rotella T) that is JASO MA rated and change it according to the owners manual (that includes the time limit)
 

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You need to use a JASO MA rated oil, in this case 10-40W is correct. This is a specification of oil that is specifically designed for air cooled engines with a wet clutch. Automotive oils will cause clutch slippage. Semi synthetic and full synthetic motorcycle oil can work but it's not worth the price since you'll need to change it more often
 

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I disagree about it being "worht it", as I've had a bike ('82 Yamaha XS400J) that I ran synthetic in, and I swapped an engine cuz I developed a valve issue, but after that engine sat in a barn over a year with no oil in it, I opened up the top, and the lubrication was such as it could have been run yesterday, I was amazed at the protection. Now I swear by it, and strangely Shell Rotella T6 5w40 that sells cheap on sale at O'REilly carries the proper motorcycle cert and is fine, tho can be more prone to leaks, but shouldn't cause new leaks, just make old leaks slightly worse. I figure though if you love these od jap bikes, and want them to last as long as possible, oil is not the place to skrimp. Hell even Amsoil or Royal Purple is affordable given the increase change intervals, and the one thing about Dino vs Synth, at change time, synth has the same viscosity, whereas dino properties will have changed significantly.
 

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+1 on the conventional Rotella-T, smoother shifting makes the case, run it until the shifting action changes then drain and change.
 

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I use conventional Rotella in my bikes but I think its more import to change oil and filters regularly than the type of oil you choose.
 

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Why I swear by T6. Was like $30 for a gallon at O'Reilly, usually on a better sale than that, still worth it though.
 

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I'll stick with the motorcycle engine specific designed oil that I use on all my bikes, Castrol 4T. It's not suitable for diesel engines like Rotella and definitely not for automotive engines.
 

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I use 15w40 shell rotella. Bike seems to like it, its certified and cheap.
 

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You need to use a JASO MA rated oil, in this case 10-40W is correct. This is a specification of oil that is specifically designed for air cooled engines with a wet clutch. Automotive oils will cause clutch slippage. Semi synthetic and full synthetic motorcycle oil can work but it's not worth the price since you'll need to change it more often
This pic from the back of a bottle of Rotella T6:

20181102_172307-2.jpg

Sellin (on a bad sale) for $30 a gallon, which is spendy but within reason IMO.
 

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Rottella T4 15-40. I get it by the gallon from Wally World for $14.
 
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