The sequence of "dots" to which you refer are called an ellipsis. Although it's common to write it as three periods
..., note that strictly it's a special typographic character
A proper ellipsis is always three dots, no more, no less.
Different style guides have different guidelines. If you are writing for a specific publication, use what is in their style guide (or trust their subeditors). If you have no style guide, pick a style and be consistent.
There are really only two options:
- Just the ellipsis on its own: "oil … that has been…"
- The ellipsis in square brackets: "oil […] that has been…"
I personally prefer the version with square brackets, since it is then clear that the ellipsis is not part of the original quote.
The Modern Language Association's style guide has changed its position, to that of recommending no square brackets.