Have some basic questions:

My Khyber car has maximum speed of 80 if I press full accelerator in 4th gear. Is that a problem? Or previous owner might have fixed that race for better fuel consumption?

What's the best way to shift from petrol to cng or from cng to petrol in such a way that both do not mix in carborator?

Can we shift from fifth gear to third gear directly or from 4th gear to 2nd gear?

When and how to change car/engine's oil? If any one can tell me for Khyber car?

I'm new to driving.. Please guide.

Thanks in advance.