yes
Only if you want to ruin your transmission.
any quality bar oil will work fine.
yes
20 in. Bar
you have to see what size the bar is.
A chainsaw can be measured by Engine size or Bar length. Both are important to the weight and cutting applications
For a manual transmission there is a fill port on the side. The plug has a hole that will fit a 1/2 inch driver from a ratchet or breaker bar.
yes
A gun sight on a chainsaw is a mark across the top and side of the chainsaw case. This mark is made perpendicular to the saw bar and on some saws is molded on the case. This can be characterized as the letter "T". The top of the T is your mark with the vertical line of the T being the bar of the saw.
An 18" or 20" bar would be ideal on a Stihl 041 http://store.chainsawr.com/
You can use several length bars on this saw. It looks like it came from the factory with a 20" bar.
The transmission fluid, or gear oil, filler hole is located on the side of the transmission. It is a round bolt just bigger than a quarter with a half inch square indentation. Use a half inch braker bar to get it loose. The drain plug is the same bolt located on the bottom of the tranny. I suggest you check your rear diff. fluid as well since you'll be under your car.