Half Life:  the amount of time it takes for one half of a given amount of material to decay

The way to compute the replacement rate required to maintain a constant volume with a given half-life is

r = ln(2) / h

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 volume.

Some rates for reference:

half life        replacement rate
2                        34.7%
3                        23.1%
4                        17.3%
5                        13.9%
6                        11.6%