Synthetic gearbox oil

[Johnhenry]

I've just drained the mineral oil from my MZ ETZ251 and re-filled with Fully synthetic gearbox oil and I must say after a few rides the gearbox and gearchanges feel much smoother.
The clutch doesn't slip as I was told it would and I may now do the same with my ETZ125.
I always use fully synthetic two stroke oil in my MZ's - and Jawa and Kawasaki Triple - and Super Unleaded fuel with an ethanol stabiliser.

[Puffs]

Ta for sharing your experience.
I just saw that I had put synthetic 10W/30 Gear oil in my YZ, but I cannot remember any significant difference with the previous oil. At least not with those boots.

Indeed, I have seen some warnings that synthetic oils can promote clutch slippage, and I also use/have used synthetic engine oils in 4 strokes, with a common sump for engine/gearbox/clutch, without clutch problems. I think those warnings are unfounded, and I'm not sure if the synthetic production method really leads to oils giving significantly lower friction than oils produced by the conventional refining process. The rheology might however be less temperature dependent, depending on the specific oils you compare.

What I use also depends on what oil I can suitably buy. Further to what I said in the other thread on 2T Oil (http://mzriders.com/viewtopic.php?f=5&t=9279), I have, in the past, run my ETZ251 on a fully synthetic 2T oil for about 20k miles, and I still lost the small end bearing. So using a fully synthetic 2T oil does not guarantee you won't get engine failure. Maybe it reduces the risk, and maybe it reduces wear, but that you pay more is a certainty. And that you get less carbon too, IMO. In my old two strokes (ETZ251 + 634) I currently use a semi synth I could cheaply buy a batch off, and that seems to work fine too (& also little carbon). In my 2T MX & enduro bikes I have always run a fully synthetic. There I choose to do whatever I can to prevent a DNF & costly repair, hang the expense. If it really helps is another matter.

[Blurredman]

Ta for sharing your experience.
I just saw that I had put synthetic 10W/30 Gear oil in my YZ, but I cannot remember any significant difference with the previous oil. At least not with those boots.

Indeed, I have seen some warnings that synthetic oils can promote clutch slippage, and I also use/have used synthetic engine oils in 4 strokes, with a common sump for engine/gearbox/clutch, without clutch problems. I think those warnings are unfounded, and I'm not sure if the synthetic production method really leads to oils giving significantly lower friction than oils produced by the conventional refining process. The rheology might however be less temperature dependent, depending on the specific oils you compare.

What I use also depends on what oil I can suitably buy. Further to what I said in the other thread on 2T Oil (http://mzriders.com/viewtopic.php?f=5&t=9279), I have, in the past, run my ETZ251 on a fully synthetic 2T oil for about 20k miles, and I still lost the small end bearing. So using a fully synthetic 2T oil does not guarantee you won't get engine failure. Maybe it reduces the risk, and maybe it reduces wear, but that you pay more is a certainty. And that you get less carbon too, IMO. In my old two strokes (ETZ251 + 634) I currently use a semi synth I could cheaply buy a batch off, and that seems to work fine too (& also little carbon). In my 2T MX & enduro bikes I have always run a fully synthetic. There I choose to do whatever I can to prevent a DNF & costly repair, hang the expense. If it really helps is another matter.

OP: How old and what grade was the old oil?


Puffs: I don't think semi or fully synthetic oils create slippage either. Which leads only to the additives. That being said- I always use 20w50 Mineral oil in my Honda, because I am sure not to get slippage.
Previously to owning the honda, I could normally stickthe same 10w40 semi i put in my car, into my bike- no slippage. I did that to the Honda.. Slippage. Perhaps the old clutch plates were of dubious age, and also the springs themselves. I have never however put anything but 20w50 for fear of ruinning the clutch, despite my replacement parts and increased strength springs.

In reality, you should really replace oils twice a year, as has been recommended on vehicles since the dawn. But who does that these days? - And.. I never did..

[Puffs]

What you're saying there surprises me a bit: a clutch slipping with 10W/40 semi synthetic, but not with 20W/50 mineral. Or maybe is it not slipping now because you replaced parts and increased the spring strengths?

In another (Suzuki) 4T I also had a slipping clutch, using 10W/40, but that stopped slipping too after changing the plates - while still using 10W/40. Maybe I also put rings below the springs to increase preload. Anyway, I think you best not use a thicker viscosity than prescribed by the manufacturer, for the conditions/temperature you use the bike in. To use a thicker oil may reduce the flow through some oil channels (towards particularly the head, OHC journals) & jeopardise lubrication when the engine is still cold.

On clutch slipping & oil viscosity: normally thicker oil will promote clutch slippage. This is because of the following: when you engage the clutch, you pull the plates apart and oil enters between the plates. It forms an oil film between the plates. Then when you release the clutch, the plates are pressed together again by the spring(s), and the oil is pressed out. A thicker oil takes longer to be expelled, and causes a softer 'landing'. And has a higher film strength, lubricates more, so that the plates have a higher chance of slipping wrt each other. This is why the YZ (+/-55 HP) uses such a thin gearbox oil, and race bikes, cars & some old MX bikes have a dry clutch.

As an aside, my wife's bike (a push rod 4T) has a clutch that has been on the verge of slipping, for about 15k miles now. When the oil is still cold, you can feel the clutch is reluctant to catch at full throttle. To combat that, & avoid having to work on the clutch, I've been mixing in a bit of 5W/40 into the 10W/40, thus reducing the cold viscosity a bit. So far that works fine.

On replacing oil: I believe replacement intervals are more dependent on use, rather than time. Because of shear under high temperature molecules break down, causing oil degradation, and the other effect is that (metal) particles have separated from the grinding surfaces & stay in suspension in the oil (and enhance wear). Both these effects are only dependent on use; oil not being used but kept at ambient temperature will keep for millions of years.

In fact, for these old bikes I recycle the gear oil. If you let gear oil stand for a few years, it becomes completely clear again. I decant it & re-use the clear fraction. This does not work for engine oil, because engine oil has additives which keep the smallest particles in suspension: it never clears. And also, (4T) engine oil has been exposed to much higher temperatures, you do not want to re-use it. Gear oil never really gets hot - unless you ride with a slipping clutch.

Brake fluid, sometimes also thought of as an oil, is hygroscopic: it absorbs humidity from the air. That water then disperses throughout the system & causes corrosion, as well as a reduction of the boiling point. That needs to be replaced on a time basis, depending on the climate you life in (normally 2 or 3 years).

[Blurredman]

I agree. You must stick with manufacture recommended, unless you know best of course

My Honda's original spec was 20w50 in 1978. Which, to be honest- was one of only a handful of oils you could have easily got. Others being SAE30 etc.

Interesting theory about the lower viscosity oil reducing slip. I only use 20w50 because I know it would work.. The engine's on 83k miles now, and at least the last 30k has been on 20w50, in my ownership. Perhaps I should put 10w40 in though? I can't decide.. When the engine is cold- and- I have to bump start it (perhaps the battery is dead), the clutch drags very much so. Infact- The clutch biting point is significantly different cold than warm- The clutch is hard and stiff when cold- light and soft when warm


My Mz's clutch slips a bit mid-rpm full throttle occasions. But.. it is due an oil change- And- seeing as I currently have 20w50 in it. I do have 80/90SAE ready to go.


As for 4t re-use over 2t. Surely 4t oil would have a lot of soot and particules extremely fine in it, perhaps that is why it doesn't split for you. After all- that's what makes it black, surely? Blowby?

[Puffs]

Engine oil doesn't clear for me because I drop it off at a recycling point.

Yes, it's partly soot and other carbon particles that makes 4T engine oil black, but also minuscule metal particles (from pistons & rings rubbing a cylinder barrel, a.o.). Gear oil in a 2T does not have soot/carbon, as the gearbox is well isolated from the combustion area. Yet that too gets black with use. Old 2T gear oil has mostly particles from the clutch plates (both metallic & non-metallic), and maybe steel shavings from elsewhere in the gearbox (if you don't have a magnet on the drain plug, as some bikes have). But if you wait, all those particles in gear oil subside, and you're left with clear oil on top. That oil has been a little bit warm, but not really hot, and if you wish you can re-use it. I do on old bikes.

But engine oil in a 4T is quite a different matter. It's not the colour that's the problem. Think for instance diesel engines with long-life oil. Their oil is clear only for the first 300 miles of their life, and thereafter it's always black, also after an oil change, due to soot indeed. Nevertheless, those engines can do 500000km - and all of that with black oil. In 4T engine oil, it's degradation due to high temperature shear that is the issue; that degrades the oil properties. So engine oil gets dumped after sufficient use, at a recycling point. And at some point probably spiked into a crude oil stream at the refinery.

PS: are you sure Honda specifies 20W/50 for the CX500? 10W/40 is quite common for Japanese bikes, also in the 70's.

[Blurredman]

The original specs when it came out stated 20w50, 10w40, 15w40