Exit from Multan, there is a petrol pump just after the exit. And yes, you can easily reach to Lahore.
BTW, petrol is available on M3 at Samundri Service area.
Follow these motorway
M5 - Sukkur to Multan
M4- Multan to Abdul Hakim
M3- Abdul Hakim to Lahore
You'll find Y junction just right after Abdul Hakim from there, one Motorway will lead to Faisalabad, Islamabad which is M4 and one will lead to Lahore which is M3.