Gearbox inspection plate

[McTrucky]

Hi complete newbie to this here... so I will start with an easy question;
The book says to fill up to just under the inspection window - that would look very full to me. I am more used to oil levels being in the lower half of an engine/box than somewhere near the top. So just sanity checking that the inspection plate is the two screw roughly D shaped window just above the gear lever and kick start shaft? (Cannot find a picture in the manual).

The manual says sae 50 which I understand is the equivalent of SAE90 gearbox oil. Any experienced opinions on what oil I should buy to replace/top up. I am aware of GL5 gearbox oils hurting certain gearboxes, so keen to know what oil would give me the benefits of a modern oil, without the risks of aggressive chemicals.

The bike is a 'lightweight' 350 from 1960 - so I think I am in the correct forum.

[ChrisTheChippy]

Hi straight 50 mineral oil will be fine in the gearbox. You do need to fill up to the level stated in the manual as the gear cluster runs high up in the box . Keep an eye on the level as they are very prone to leakage and if the oil level falls too low premature wear will be the result.

[ChrisTheChippy]

Sorry forgot to add yes the inspection window is the D shaped cover at the top of the box.

[McTrucky]

Sorry forgot to add yes the inspection window is the D shaped cover at the top of the box.

Perfect. Thanks.

[McTrucky]

Just ordered some Smith and Allan SAE50 non detergent classic oil. Same oil for engine and gearbox keeps things simple. 5l for £27 including delivery is fine by me.

[SPRIDDLER]

The manual says sae 50 which I understand is the equivalent of SAE90 gearbox oil.

Welcome to the Forums.

Yes, their viscosities are the same. However, thy differ in the additives.
Briefly..........
Gear oil has particular additives for shear forces and shock loading (e.g. hypoid differential gears which our gearboxes/transmissions don't have), anti-foaming, rust and corrosion prevention etc.
Engine oil has additives for high temps, combustion by-products etc.

I use SAE 50 engine oil in gearbox and engine as recommended in the manuals. (Fine for our relatively low power unstressed engines).
In my opinion modern multigrade may be beneficial in twins since on start-up it is important that the shell bearings receive a quick supply of oil under pressure, plus a cold thick oil on start-up can result in over-high pressure causing joints to leak.
The singles have ball and roller bearings and an immediate supply under pressure isn't so critical as they will have a residual hydrodynamic film coating. This is why the singles can have oscillating pumps (as animation below) delivering low pressure pulses of oil and the twins need the rotary gear high pressure continuous flow pumps.

Oil pump action..gif

[McTrucky]

Perfect - thanks. I was second guessing gear over motor oil and assuming gear oil had to be better for gearboxes - but very happy to have the same oil in both. Makes life simpler.

[Red Gauntlet]

I have always used SAE90 in the gearboxes never a problem in over 40 years, the advantage is that you can tell where the oil leak is coming from by the smell. Unless you have feral cats that is The F. Neill manual states you can use either.