I knew about the Oberth effect but I can't put it into action. Every time my transfer window is not at the Periapsis, I would just say a bad word. Thanks to your post, now I can actually put the transfer window into my own hands instead of just leaving it to luck. For a guy without the expansion(me in 6/12/2019) I need to take every possible action to save fuel and your post just helped me a lot. Merci beaucoup.