80w-90 gear oil in a 2T moped

[Japanese]

Hi everyone,
I have vintage 50cc 2 stroke moped that requires EP90 gear oil for the gears that share the same oil with the wet clutch.
I was thinking initially to get some 10W40 or 20W50 motorcycle oil, but decided to follow the manufacturer requirements because that thing has a small engine and it gets very hot.

So I got Motul Gear oil 80W90 (mineral) as the closest to EP90 which I didn't find (Castrol has it for vintage cars, but saw it too late). The Motul oil has MoS2 (molybdenum bi-sulfate) and is high viscosity oil KV100*C - 21.7 cSt.

My question is:
Do you think the wet clutch would slip as that oil has moly and probably other friction modifiers?

Motul 80W90 Gear Box - tech data
https://azupim01.motul.com/media/motulData/DO/base/GEARBOX_80W-90_en_FR_motul_35100_20210810.pdf

Thanks.

 

[GR_CoB]

Hard to say. Not many mopeds around here, but many cases of different motorcycles (with shared clutches) reacting differently to the same oil: some slip the clutches right away, some start to slip after a couple thousand miles, and rest don't seem to care and have no slippage ever.

Motul does seem to meet the "Extreme Pressure" spec of the EP90, so most likely you're good. Another good option would be this:

 

[GSCJR]

If it were me, I'd look for a GL-4; Red Line MT-90 comes to mind.

 

[Japanese]

Thanks both for the quick reply.
For the reason the moped is vintage I would like to stick to mineral oil. And yes, it says the Motul is API GL-4 oil.

 

[Japanese]

I decided to use 10W-40 or 20W-50 the thickest. Also this moped was driven with SAE 30 until now.
The 80W-90 is way too thick for this 50cc moped at 21.7 cSt (KV 100°C). Plus the MoS2 for sure would cause clutch slipping. I'll give the 80W-90 to somebody who uses farm equipment.

 

[Bonz]

It sounds like 30 grade worked well in the moped, what's the thought on changing now to one of the grades you mentioned?

 

[BusyLittleShop]

Gear Oil Viscosity...
Because the viscosity of gearbox and engine oils are measured on
different scales, an SAE 80 gear oil is roughly equivalent in
viscosity to an SAE 20 grade engine oil... and SAE 85 gear oil is
roughly equivalent to SAE 30 grade engine oil...

Clutch...
Friction modifiers additives are only a small percent of the total oil
product and help the base oil do things that it otherwise could not...
Additives fall into several basic categories but Moly, Phosphors and
Zinc are the most often used friction modifiers... what ever small
percent of FM employed they will not defeat a wet clutch in good
working order... mileage not additives are the root cause of our
clutches to loose grip... Under scrutiny you'll find that your slip
was due to normal glazing and contaminates...

 

[Japanese]

It sounds like 30 grade worked well in the moped, what's the thought on changing now to one of the grades you mentioned?

Factory recommendation for it was EP90, back in the late 60s. It was run with SAE 30 as easiest and cheaper available. It looks like nowadays the gear oil for scooters and mopeds is 10W-40.

The moped is air cooled and runs always hot. Also we had a gear failure back in the days. It has a small engine cases and engine heat is quickly transferred to the gears as well. It doesn't have very good cooling.

 

[Japanese]

Gear Oil Viscosity...
Because the viscosity of gearbox and engine oils are measured on
different scales, an SAE 80 gear oil is roughly equivalent in
viscosity to an SAE 20 grade engine oil... and SAE 85 gear oil is
roughly equivalent to SAE 30 grade engine oil...

Clutch...
Friction modifiers additives are only a small percent of the total oil
product and help the base oil do things that it otherwise could not...
Additives fall into several basic categories but Moly, Phosphors and
Zinc are the most often used friction modifiers... what ever small
percent of FM employed they will not defeat a wet clutch in good
working order... mileage not additives are the root cause of our
clutches to loose grip... Under scrutiny you'll find that your slip
was due to normal glazing and contaminates...

Thanks a lot! This is very helpful.
Yes, I was mainly guided by this chart which I found on here.

 

[BusyLittleShop]

Thanks a lot! This is very helpful.
Yes, I was mainly guided by this chart which I found on here.

You're welcome...