Gearbox oil

Hi,

I changed the Gearbox oil from Eni Rotra MG/S SAE 85W-90 to Motul 300 75W90, but it seems the Gearbox is more "Clunky" and noisy now....
What is your experience with this?
Is there an oil out there that can smoothen up this rough Gearbox?

Thanks

I can't get a lot of viscometric data from the specs and technically they both meet similar high temperature SAE 90 viscosity spec (<18.5cSt at 100C....anything <11cSt would be an 80 grade, so you can drive a bus through the range of 100C viscosities possible to meet the spec). They aren't hugely different at 100C (what's 3 cSt between friends !)  BUT Motul 300 75W90 spec says its viscosity is 87 cSt at 40 C; and ENI Rotra MG/S 85W90 spec has viscosity at 200 cSt at 40 C.

I'm guessing that when the gear oil is cold (like room temperature up to 60C); the Rotra MG/S is double the dynamic viscosity of the Motul 300 & that's why you 'feel' the gearbox is clunky & more noisy with the lighter (viscosity) oil.

At 100C the Motul is 14.2cSt versus 17.5 for the Eni Rotra; so less of a difference, but you may still 'feel' the effects of lighter viscosity at high temps


To answer your question, buy gear oil that is same or higher viscosity at 40C to 100C range as the Eni Rotra MG/S & your gearbox won't be clunky & noisy.

fix in the old days (mini gearboxes, Morris minors, rear differentials) if you wanted to sell a noisy gearbox car was to stuff the gearbox/diff with a few pairs of stockings and sell it quick while the viscosity stays way up high !

Thanks Paul.
I have a bottle of Eni Rotra MG 85w140 laying around in the garage, but the vicosity is the double...No-Go, right?
Eni Rotra MP/S 85 W 90 @ 40ºC--> 200 mm2/s ; @ 100ºC --> 17,5 mm2/s
Eni Rotra Bike MG 85w140 @ 40ºC--> 410 mm2/s ; @ 100ºC --> 28,2 mm2/s

BTW I use Motul 300 75W90 ...I was just explaining why you might notice a difference in noise - I don't think it's noisy but then the whole engine is a noise machine

.....AFAIK 75W90 is fine & probably 85W140 is fine as well but I'd personally stick closer to the recommended specs

The ENI Rotra MP/S 85W90 is very hard to find, so I just ordered the LIQUI MOLY Hypoid GL5 85W90 because it has similar viscosities at 40ºC and 100ºC.
It's a mineral oil, so I expected the Motul 75W90 (100% synthetic) to be a better option...
Fact is, I changed from Rotra to Motul two weeks ago, after that I made a 1.000km ride, and the difference in the feeling of the gearbox was noticeable.
It felt "dry"....The "clunck" while shifting was very anoying...sometimes I even avoided shifting...
I must say that the Rotra oil, after aprox 26.000 km on the bike, looked pretty good when I drained it (differently from the bevelbox oil that looked nasty...).
So now I will be running Motul 75W90 synthetic on the Bevelbox, and Liqui Moly 85W90 mineral on the Gearbox

To be honest it's hard to find any full synthetic gear oils 75W90 high temp spec that will be much higher viscosity than the Motul 300 (The Eni Rotra MG/S is just blended up to higher viscosity & is 85W)...it's simply the natural viscosity Index of synthetic oils that means it hits the 75W at lower temp & 90 at higher temp end.....

Some people (as far as I remember) have gone for Redline 75W110 because it's a bit thicker & reduces noise & it isn't as thick as SAE140 grade. In fact it's 'only' 2cSt KV@100C than the Eni Rotra MG/S & what's 2cSt between gears'? OPIE sell it in UK for GBP 26/L....so it's about GBP10 more per L than the Motul 300

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If I came across that Red Line oil I might have given it a try...
But know the Liqui Moly 85W90 it's on the way...
I will give some feedback when I try it.

Hi,
Just did my first ride after changing the gearbox oil to Liqui Moly 85W90 (mineral...).
It's really noticeable the difference in shifting. So much smoother...
I guess these agricultural gearboxes were not made to use full synthetic oil...
Man...I'm tempted to try Yak fat like Pete mentioned in another post .....