I don't know why you'd want to go back to regular oil?
Do the "Seafoam" treatment to help rid yourself of the combustion chamber surface deposit build up, your tired o2 sensor caused. The is fun to do, & will immediately improve throttle response & idle. But don't add to oil
, as you already changed your oil. 1/3 in the power brake booster Vac hose, while at warm idle. Shut off engine & and let sit for 20 minutes. Then restart & blow out the crap. The other 2/3 can goes in the gas tank. This will improve fuel atomization by the injectors. Safe for your new o2 senor & your Cat converter too. http://www.f150forum.com/f33/how-sea...-1991-a-31505/
Here''s a Vid that another Forum member shot when he did his truck this weekend. So you'll know how much smoke to expect when the crap starts to burn out of the chamber surfaces. http://www.f150forum.com/f2/seafoamed-today-58880/
And spend a couple extra bucks & buy "brass" cap & rotor. It's actually cheaper, as they will outlast the stock versions two to one. You will also need them if you decide to do the Ignition system upgrade that is very popular around here w/ the Forum members.