Real quick guys:

1. Been using Mobil 1 for approx. 60k miles - have since sprung a leak (perhaps rear main seal?). Can I switch to a Valvoline Semi-Synthetic (high mileage oil) with no problems?

2. I have been getting a constant on SES with no pressure when I release the gas cap. I believe I may have three problems - bad fuel pump (ran it low WAAAAy too many times before I found out it was not good to do so), bad gas cap (not getting a good seal) and old fuel filter (last changed 40k miles ago), am I on the right track?

Thanks

 

Don't worry about changing oils, you will be fine.
I'd replace the gas cap, or fix it so you get a good seal, and let it go for 60 miles. The ses light will go on if the cap isn't put on right, and without a good seal, I guess it would be the same thing. I believe it takes about 60 miles, it could be less, but it shouldn't go off right away. Try these first before the fuel pump, I'd go ahead and change the filter if you get a chance, it probably needs it anyway.