My 1999 Subaru Impreza sport wagon had a misfire in cylinder 2, after I changed all four spark plugs and fuel filter the problem got worse. Now the throttle lags and misfires only happen when the car accellerates from 0 to 20, then the problem disappears. When I was changing the spark plugs, there was also oil completely coating the outside of the rubber gaskets that plug over the spark plug, is this due to a cracked valve cover gasket?

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Hope this helps

check your grounds. I find many easy fixes by cleaning all the grounds around the engine bay. I would also check the ground at the fuel pump connector that wiring is prone to corrosion and breakage

Open rear liftgate and there are little slide clips on each end, you pull the clips then slide gas strut/support cyls off. Most new replacements come with new clips.

That means that the fuel being injected into cylinder 1 is not being ignited at the proper time.

Check your spark plug wires for damage and make sure that the plug boot or coil assembly is seated properly on the plug.

If all of the above are correct then it needs to have the spark plugs changed.

check you ecu. same problem happen to me in an izuzu rodeo. I let my fuel get to low and it sucked dirt into the filter and messed with that and it set off a code for the ecu and messed it up but if i had just replaced the ecu first i wouldnt have had to change the injectors or anything.

This is most likely detonation (aka pinging, or pre-ignition knock), a condition caused by the fuel igniting before the spark occurs at the spark plug. The "knock" sensors in the heads are designed to control this effect but sometimes if you get a fill of bad fuel (fuel with a low octane rating), the sensors are unable to cope. Other causes are...
1. Bad spark plugs
2. Build up of carbon deposit in combustion chambers
3. Turbo boost set too high
(a) Avoid (at all cost) hard acelleration until all this fuel is used
(b) Add a fuel octane improver such as Mythyl Benzine
(c) Quickly get a fill of premium unleaded fuel.

I would do that plus test the plugs, wires and coil pack.

1. Warm the engine up, Remove the Drain plug and drain oil out (Drain plug is on the drivers side of the pan).
2. When oil is done draining replace the drain plug and install a new crush seal (silvery washer on drain plug)
3. Next you will need a Oil filter socket. Use the oil filter socket with a 3/8 drive ratchet with a 6 inch extension, remove the filter, it will be full of oil.
4. Use fresh motor oil and coat the gasket on the new filter and install. tighten filter 1.5 turns past hand tight.
5. Refill the engine with 5w-30 Conventional oil. (start with 4 quarts than check the dipstick) Subaru recomends havoline as this is what they come with from the factory.
6. Wipe up any oil that may have spilled on the exhaust cross over pipe.
7. Start engine check for oil pressure and leaks.

First off you want to make sure that you have the correct firing order down. When you rearranged the spark plug wires you might have made the firing order off and that could cause the misfire to get worse. the things that you should do to isolate the problem is to perform compression tests, fuel injectors, and also check the entire ignition system

When cleaning your engine, water/degreaser went down the spark plug ports. Try using a air compressor blow gun to remove the moisture from around the spark plugs and boots. The water was causing the engine to misfire on 3 cylinders at first and as the ports started to evaporate the moisture, the less misfiring was occuring. Dry thouroughly and hopefully coils do not have damage to them. Also check to make sure the boot is making good contact with the spark plug.