I just changed my oil. I couldnt fing 20/40 in synthethic so i Used some 10/40 synthetic mobil 1. Is this alright even though the manuel reccomends 20/40. My bike is a 1990 xt 600. thanks for your input

The wonderful world of oil.

The most important factor in oil is the viscosity. This is a number which indicates the relative thickness of the oil. The higher the number the thicker the oil.

Unfortunately thin as well as thick is not good. Thick oil is hard to pump through the engine and does not make a nice film. So thinner oil is better. But... when the oil is to thin the oil-film (which is what it's all about) might rupture or break, leaving the bare metal unprotected.
A difficult problem, how to choose?

The sae number give's you two (actually three) pieces of information:

The first number is the base viscosity. Usually followed by the letter W which means this viscosity is at 0 Fahrenheit or 17 Celsius. (so, when the engine is cold)

The second number (behind the W) tells you what the theoretical viscosity would be (cold) for an oil that is as thin (or thick) as this oil at 100 Celsius. So, the higher this number the less thinner the oil will get when it heats up.

Conclusion:
If you have to deviate from the manufacture’s proscribed SAE, the first number is the most important one. If you have to use 10W40 and you would use 20W40 you can expect problems with the clutch and excessive engine wear when cold.
On the other hand, if you choose an oil like 10W30 the oil might get to thin and break up when the engine is hot.
In the case of choosing 10W40 instead of 20W40 you have an oil which is thinner when cold, but not to thin when hot. So.. it’s even better then the original.

One extra interesting piece of information is the fact that manufacturers service intervals are determent for mineral oil (or dead dinosaurs juice). Synthetic oil will last 2 or 3 times this period.

Maarten

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maartensworld.com
beyond-anywhere.com

the manual recommends mineral oil.
of course you can use synthetic oil and ...40 ist o.k., but in my opinion it is not necessary for normal use.
main thing on oil is the API classification and not the brand. refer to your manual and use oil according to this or better (mineral or synthetic).
but you should use an oil designed for motorcylces because here the oil has to serve also the gears and the clutch (which is not the case in a car engine).
regards
klaus

...and, of course, what mmarten said about the viscosity is right. use viscosity according to the manual or better, e.g. 10W50 or 5W60 (synthetic).
klaus

So what i am doing is alright and will not cause any damage. I have notice that the clutch opperates much smoother. and it seems to be easier and doesnt clunk as much as with regular 20-40 oil.
Thanks for your help