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any quality bar oil will work fine.
yes
20 in. Bar
you have to see what size the bar is.
yes
A chainsaw can be measured by Engine size or Bar length. Both are important to the weight and cutting applications
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.
Not satisfactorily. The 021 is a lightweight saw.
The Stihl 032 was introduced in 1978 and the maxrecamended bar length is 20". But I know from experiencethey can easilyhandle up to 24".