I have city 2004 idsi.
1. For engine oil, the recommended one is 5W - 30, if there is no oil burning issue.
2. Doesn't it have 42 litre tank? Moreover, you are getting very less fuel average. From L12, L13 engines in manual transmission, fuel average in city is around 14-15 and on highways 17-18 eadily. Get it checked from any good efi electrician. I had same issue and it fauly tp sensor was diagnosed.
3. For reservoir caps, you may search online. Easily available