'weight' of oil

[Euan Uzami]

read something on the internet advising the use of 'medium weight' oil.
What's an example of a 'medium weight' oil?

I presume the advantage of heavier oils is that it stays around longer, so what's the disadvantage of it? Stickier, attracts more gunge, more friction? (Attracting more gunge would be why not to use it on the chain?)
I've been advised to use GT85 on the rear derailleur pivots rather than TF2 because it's lighter.

A good example that highlights the difference on my bike is pedals - if I don't lube the pedal/cleat interface pretty much every  ride, they will definitely creak - I put a little bit of car oil on them on saturday and they still aren't creaking (fingers crossed...)

[Biggsy]

Lighter oils allow components to run more freely, but things like derailleurs still work fine with heavier lube and are in fact supplied with grease on the pivots.  A lighter lube penetrates more easily, though, especially if it contains solvent, like GT85 does.  That might be why you're being advised to use it.

I don't like using GT85 on the bike (especially in spray form) as it can disolve grease and too easily go where you don't want it to.  I reckon a general purpose oil - like the stuff sold in Wilkinsons - is easily good enough for derailleurs.  I'm not sure if that counts as medium or light oil.

[matthew]

The 'weight' of the oil referse to the fraction from the distilation column. A light oil has a lower boiling point than a medium or heavy oil. Typically this means that a light oil is less viscose (thinner) than a heavy oil and it iwll therefore penitrate further into a mecanism. However it will also be squeased out from the contact points and may fail to lubricate the components properly. Conversly a heavy oil will remain where it is put but may not penitrate to the critical point of the mechanism.

oils tend to be sold by 'weight' light oils being of the order 5W-20W and heavy oils >75W. Modern synthetic car oils are mixtures and sold as eg 5/20W or 10/40W. The point being at low temperatures the lighter components penitrate to the required parts then as the engine warms up and the light components become too thin the heavier weights has become suitably runny to reach those same parts.

[rogerzilla]

Beware though - the SAE viscosity numbers are on an arbitrary scale, and you can only compare them within the same type of oil.  For instance, 10W40 motor oil is about the same viscosity as 75W90 gear oil.  They are on different scales so that no-one spots 10W40 motor oil on the shelf and puts it into their car's gearbox.

If you do a quick google you can find the SAE numbers against actual viscosities in stokes, poises or whatever units you fancy.

By the way, the "W" stands for "Winter" in a multigrade oil, and its derivation is quite complex, intended to show how it performs under real cranking situations at low temperature.

If we're talking about straight single-grade motor oil, SAE 30 is pretty medium.  It's sold for garden machinery.

[bikenerd]

Also, suspension fork manufacturers just make up the number they use for the weight of oil, and usually renumber it from the generic brand of motorcycle fork oil they are actually using.  Of course, a 5W oil is lighter than a 10W oil from the same fork manufacturer.
However, Rockshox 5W oil is lighter than Fox 5W (IIRC).  You can Google for conversion tables.