91 Camry, Engine runs rough

Morning folks.

I've got a 91 Camry, 3SFE engine (4 cylinder) with 422K+ miles on it (the odometer doesn't turn the way it used to).

Needless to say, the car is a bit rundown, and cost me nothing, but I am having a lot of fun playing around with it, and believe it or not, it used to run reasonably well.

Recently however, the engine has been running rough. To symptoms. Occasionaly on the highway it feel like it hiccups and the car jerks. More recently it has had an uneven throaty feeling to it, and doesn't produce as much power.

This is especially true when accelerating or going up steep hills.

I checked the air filter which and replaced it. I replaced the plugs as well. The #4 plug (facing engine from hood on far right) was heavily oil fouled, #1 slighly fouled.

Because this is a relatively new roughness and loss of power, was going to check the distributor cap, wires and rotors next, followed by the fuel filter and perhaps the vacuum system? Also going to go to 10W-40 oil, but the oil looks pretty clean otherwise.

Any areas that I should obviously focus on? I'm kind of enjoying keeping this car alive, and would love any extra help.

I was also curious how much gas would come out when replacing the fuel filter.

Thanks for any help!