Congratulations Gull, for buying this mean machine. This is one of the best available machines in the market. The thing I like the most about it is its engine with oil filter, that surpasses it from its so called competitors. I was also in love with it once, but didn't buy it due to some issues. BTW have you read its owners manual. It was already available here on the net.
I have uploaded it on dropbox.
https://www.dropbox.com/s/8ulgaequonfqtu8/Suzuki-GS-125-150-OM-ur.pdf
Please read on page 10. I think that its engine oil change interval could easily be stretched to 4000 kms as mentioned in the maintenance chart. I dont have one but I will definitely try it with the fully synthetic oil up to 5k kms, if I had one.
Bravo. A unique characteristic that's why I give its engine 10/10 for long life and reliability.