To replace lifters on 1994 1.8 EA82 manual, subaru--

Yes, that's entirely possible, especially if it has hydraulic lifters. They sometimes get plugged and then don't function properly. And they are right in the top portion of the motor.

never heard of it
10w-30 is too thin for an old engine
try 20w-50 and make sure that a quality oil filter was used

This can be a sticking Intake valve or bad valve lifter Sometimes the oil seal in the Valve stem gets varnished up and the valve will not move freely. When the valve is pushed down by the Rocker arm, the valve is too dirty to come back up all the way. You can also have a weak valve spring. Either way, part of the compression is leaving the cylinder and going backwards into the Intake manifold. That is the popping noise you hear.

A valve lifter which is clogged up can also do this. On some engines a single valve lifter can be changed without taking off too much. Some engines allow for a tool to go through the pushrod hole to swap the lifters.

The reverse flow can sometimes leave a trail of carbon inside the Intake manifold. If a mechanic takes that part off, sometimes the cylinder can be identified and worked on separately. There are procedures which allow for even a valve spring to be changed without a total teardown of the heads.

You can also try a product called Risolone or similar to dissolve varnish in the oil passages and the valve stems or clean lifters. Then change the oil.

A less likely problem is a loose timing chain creating slack in the timing process. The valves are not completely closed when the engine fires This allows the compression to escape backwards through the Intake manifold. Although it might seem this would also happen when the engine is warm, the Electronics change the timing when the coolant temperature rises so you can easily have a different timing setting for either a cold or hot engine.

Its worth a try to attempt to clean the interior of the engineby dissolving varnish. I hope I have been very helpful in explaining what your problem is.

I find it hard to believe that it runs fine but, to replace either, you need to remove the intake manifold. Likely you will find both items in the lifter galley. Before dropping a lifter in there, make sure the cam lobe is still good....If the old lifter is severely worn on the cam end, likely it's not. After replacing the parts, but before replacing the valve cover and manifold, turn the engine by hand and make sure that the valve opens and rocker travel is the same as all others.
The r&r is covered in haynes and chilton manuals. One tip is to always use washers on any bolt that attaches the manifold to the heads to spread the torque load and yes, always torque any aluminum manifold to prevent warpage..
If either part has fallen into the oil pan (unlikely) it actually can stay there forever without harming anything.

YOU HAVE TO REMOVE VALVE COVERS LOOSEN REMOVE ROCKER ARMS ASSEMBLY KEEP ROCKER ARM ASSEMBLY IN ORDER SO THEY GO BACK IN THE ORIGINAL POSITION.YOU HAVE TO REMOVE INTAKE MANIFOLD ALSO.YOU NEED A REPAIR MANUAL FOR TORQUE SEQUENCE ON THE INTAKE MANIFOLD AND ROCKER ARMS TORQUE.

Under the upper intake manifold on the drivers side

The EA82 does not use shims. Ticking almost always caused by a failed oil pump to engine block gasket failure. The gasket slowly fails which then allows the oil pump to pump air along with the oil. Replace the "Mickey Mouse" oil pump gasket.

the lifters are inside the engine under the intake manifold, you must remove the intake manifold, valve covers, rocker arms, and push rods. do you have a repair manual? I suggest Haynes or Chilton, if you do not have one check your local library, they may have one

Remove valve covers, remove rocker arms and push rods, remove intake manifold, remove lifters, (special lifter tool may be required). Reverse steps to reassemble. Make sure to use new gaskets. 

best thing to do is to take the intake and valve covers off, its possible you may have inadvertently sucked something up and it's in you intake. That is the best solution.

No telling what it is until you take things apart and take a look. It shouldn't be too hard, make sure you get new gaskets, gaskets are cheap anyways.