Half Life: the period of time it takes for the amount of a substance undergoing decay to decrease by half
The way to compute the replacement rate required to maintain a constant volume with a given half-life is
where ln is natural log, h is the half life and r is the replacement rate required to maintain a steady state.
So, in order to maintain a given volume of oil with a half life of three days and a continuous rate of replenishment, the effective daily rate must be ln(2) / 3 or 23.1% of the total steady state volume.
Some rates for reference: