As we all know, Servants consume large amounts of Mana when they fight. They gain part of it through the Holy Grail, and the other part is normally filled by the Master - however, if a Master lacks the Mana to support a Servant, and that Servant uses up all magical energy he/she has, they'll disappear forever. But even with normal humans or a weak Magus as the Master, all Servants have other ways of regaining their strength, and filling up their capacity. Remember, if your Magus lacks the production of energy the Servant needs, you'll HAVE to use one of these ways to be ready for battle!
This is slightly straying off of the usual FSN canon. The 'bodily fluids' a Magi, be it female or male, produces during sex are highly charged with Mana, even for a weak Magus - sex with a Servant can indeed recharge their Mana. However, some things to keep in mind:
a) all Threads with such content have to be labeled with a [M] in the title.
b) just some 2 post uninspired, badly written smut/lemon won't get your Mana recharged. Staff will judge wether or not the recharging was successfull.
c) keep things in Character, even with this system. If you start to go at it like rabbits, you'll be punished.
Remember that this method only works for Magi, or people that at least have the potential of using a Magic Circuit. Normal humans can't 'recharge' a Servant like this.
Servants are spirits, no exceptions - there will be no second case of Saber from the original Visual Novel. They all are able to steal life energy and even whole souls from humans to recharge their mana, but be careful if you do this: Killing humans without any holding back will not only cause any decent Master and maybe even Servant to hate you, but also make you a target of the Magic Assosication and the Church! In other words, Staff Members may decide to use special Staff Characters with enhanced powers to obstruct you.
Touch the Grail
A Servant that is able to touch the haywire Holy Grail will be able to recharge their own mana to the absolute maximum. However, touching the Holy Grail is only possible if you've killed at least 6 other Servants, and obtaining it won't be possible for a while. In other words, don't count on this too much.