Yes this is normal. Remember the higher the resolution the more information Ps has to work with, and visa versa, the lower the resolution the less Ps has to work with and the effects will be much more noticeable. Also remember lighting and color play a role in there as well.
The Oil Paint filter settings are going to vary with each image you work with. You may have to raise/increase the settings, but as long as your ultimately getting the desired effect, then no worries. The same holds true for the sharpen filter. However, you can just as easily take the higher res/sized image and make it smaller before starting your project if you prefer the effects on the smaller image.
It's just a matter of experimentation. Not sure what caused you to try the sharpen filter, but somehow you figured out it worked for the desired effect you were searching for. As for mine, I was using two images, one before the Oil Paint filter placed under the second one with the oil paint filter. I then applied a layer mask to the second (one on top) and used the Brush Tool set to black and a lowered flow rate ( 3%) to bring back the detail. But hey, it's about what works, right?
Gotta run to bed now, catch you later!