Hello again everyone 
I did my first oil and filter change myself, all alone on my own, and it was actually really easy.
I'm now trying not to think too much about all the times in the past I paid a mechanic to do it while I could have done it myself and saved money
When I bought it, 4-5 month ago, in July, the guy who sold me my Kawasaki GPZ 500S 1997 told me he just changed the oil and the filter.
I've read the bike's official user's manual and they recommend to change the oil and the oil filter every 10.000 km (6200m), so I thought I still had a lot of time.
However, my bike has a very annoying problem: The oil level sight glass has an opaque layer on the inner side that prevents me from reading the oil level the normal way.
Here's my oil level sight glass:
As you can see, impossible to read any level (and don't get fooled by the black thing that looks like an oil level, it's just part of the rubbish stuck on the inner side of the window).
I describe the full problem in the post "Unreadable oil level sight glass gauge on Kawasaki GPZ 500S 1997" in this very forum (sorry I cannot link it directly I don't have enough post on this forum yet
)
I have tried to scratch gently the inner side of the glass with a wooden chopstick inserted in the oil refill hole, but it's unreachable.
Therefore, I decided to change both the oil and the filter long in advance, less than 3000km/2000 miles, after purchase, to make sure that the oil is new and that there is the recommended amount of oil in the engine.
I just did that and expected to drain about 3 litres of oil from the engine and filter, since it's the recommended level.
But turns out that I only drained 1.8 litres, so there was waaaaay less oil than it should.
In addition to that, the oil was really opaque, so I think the oil and filter change was a lie.
I think I'm lucky to have decided to change the oil now, otherwise I would have probably seized the engine if I had waited the 6200 recommended miles trusting the seller naively.
I have refilled the engine with 3 litres of oil and a new oil filter as instructed in the user's manual.
I intent to change the oil again in advance, but this time I will pay a mechanic to do this and remove the side of the engine, replace the oil sight glass to be able to read the level correctly and proceed to the next oil and filter change all in one go.
I have several questions about all this:
Can there be bad consequence of having used my bike with used and insufficient oil? (After the oil change I took it on a few rides of a few dozen miles and everything ran smooth and nice).
Is there another way to control the oil level without the oil level sight glass? (I thought about using a chopstick as a dipstick inserted on the oil refill hole and use the oil mark on it as a gauge, like on cars. What do you think? Do you have another technique?)
What is, in percentage or in litres, the difference of quantity of oil between MAX level and LOW level on the oil sight glass?
Without leaks how much oil needs to be added every 500 miles or so? (I have no idea of how much oil is consumed during use)
Thank you for you help guys.
I did my first oil and filter change myself, all alone on my own, and it was actually really easy.
I'm now trying not to think too much about all the times in the past I paid a mechanic to do it while I could have done it myself and saved money
When I bought it, 4-5 month ago, in July, the guy who sold me my Kawasaki GPZ 500S 1997 told me he just changed the oil and the filter.
I've read the bike's official user's manual and they recommend to change the oil and the oil filter every 10.000 km (6200m), so I thought I still had a lot of time.
However, my bike has a very annoying problem: The oil level sight glass has an opaque layer on the inner side that prevents me from reading the oil level the normal way.
Here's my oil level sight glass:

As you can see, impossible to read any level (and don't get fooled by the black thing that looks like an oil level, it's just part of the rubbish stuck on the inner side of the window).
I describe the full problem in the post "Unreadable oil level sight glass gauge on Kawasaki GPZ 500S 1997" in this very forum (sorry I cannot link it directly I don't have enough post on this forum yet
I have tried to scratch gently the inner side of the glass with a wooden chopstick inserted in the oil refill hole, but it's unreachable.
Therefore, I decided to change both the oil and the filter long in advance, less than 3000km/2000 miles, after purchase, to make sure that the oil is new and that there is the recommended amount of oil in the engine.
I just did that and expected to drain about 3 litres of oil from the engine and filter, since it's the recommended level.
But turns out that I only drained 1.8 litres, so there was waaaaay less oil than it should.
In addition to that, the oil was really opaque, so I think the oil and filter change was a lie.
I think I'm lucky to have decided to change the oil now, otherwise I would have probably seized the engine if I had waited the 6200 recommended miles trusting the seller naively.
I have refilled the engine with 3 litres of oil and a new oil filter as instructed in the user's manual.
I intent to change the oil again in advance, but this time I will pay a mechanic to do this and remove the side of the engine, replace the oil sight glass to be able to read the level correctly and proceed to the next oil and filter change all in one go.
I have several questions about all this:
Can there be bad consequence of having used my bike with used and insufficient oil? (After the oil change I took it on a few rides of a few dozen miles and everything ran smooth and nice).
Is there another way to control the oil level without the oil level sight glass? (I thought about using a chopstick as a dipstick inserted on the oil refill hole and use the oil mark on it as a gauge, like on cars. What do you think? Do you have another technique?)
What is, in percentage or in litres, the difference of quantity of oil between MAX level and LOW level on the oil sight glass?
Without leaks how much oil needs to be added every 500 miles or so? (I have no idea of how much oil is consumed during use)
Thank you for you help guys.