I guess my bit of advice would be to know your stuff, but don't try and show off. I would guess that the examiner is probably most interested in hearing about the hydraulic gear system, but you can always ask for clarification, "Do you mean the hydraulic gear system?" Don't go and answer a bunch of questions that weren't really asked.
A fairly general rule with orals is to answer the questions well enough so that you don't sound like a big dummy, but don't give them enough rope to hang you. If you start to go into a bunch of unnecessary detail and you start to seem unsure of yourself then you know you're going to get more questions on that subject.
I wouldn't get into numbers/details unless I'm asked. I would make sure I knew them, just in case, but I would keep my answers short and simple. If they want to know more detail, they will ask. And if you don't know an answer at least know where to find it in a reasonable amount of time.
Also, alot of times these examiners are really good guys/ladies and they want you to do well, but many of them are very experienced and they are happy if you can learn something from them. Sometimes they will ask questions that seem ridiculous like "how many bolts connect the wing spar to the cross over box?" They're probably just having some fun with you, but you might actually be able to learn something from them.
Never make them look dumb, or your oral will become much more difficult. Remember they are supposed to be old and crusty and you are still wet behind the ears. If you are confident but humble and don't seem arrogant you will score big points.
Above all it's their job to make sure you're safe. There's nothing more scary than a pilot who thinks he/she knows it all. No one does. And just when you think you do, you'll realize that you forgot the first half of what you learned. I think it's true when the crusty old guys say "I've forgotten more than you now know." It doesn't mean that they just have bad memories.
Good Luck!
P.S. I suppose you could design a hydraulic system that uses maple syrup if you really wanted. Probably wouldn't work too well, but it'd make the walk arounds alot more fun. "Hey you! Quit licking that airplane!"
