With the edits you've made, all of the sentences are factually correct and convey the meaning you were looking for. I would suggest adding a comma after 'In 2014' in all cases, so you would have:
- In 2014, 10 more tonnes of gas were sold compared to oil.
- In 2014, the amount of gas sold was 10 tonnes more than that of oil.
- In 2014, the amount of gas sold was twice the amount of oil.
As to which to choose, it really depends on what information you want to focus on. The final sentence emphasizes the dramatic difference in sales between the two products. The first two sentences emphasize the 10 tonne difference,which could be a major difference or a minor one, depending on how much oil was sold.
Two of the sentences are correct, but one changes the meaning of the data.
In 2014 20 more tonnes of gas were sold compared to oil.
This sentence would mean that the following equation would be true:
gas sales - oil sales = 20 tonnes
That isn't the case. According to your data, the answer to that problem would be 10 tonnes. You could say something like this, though:
In 2014, 20 tonnes of gas were sold. This compares to 10 tonnes of oil sold.