When I join a new ship one of the first things I do is to find the the normal working capacities of all the oil systems, including hydraulic systems, cranes etc, and list them under the grade of oil they use.
I then look back through the records to see what the average monthly consumption of each grade of oil has been.
I then check through the oil companies handbook to find out what the cheapest delivery Ports are and what are the minimum Bulk Delivery quantities and the minimum quantity of oil in Drums that will be delivered Free of Charge (usually 4 x 205 litre drums) but these quantities vary from one oil company to another and from one part of the world to another.
I then estimate how much of each grade of LO we will have onboard at arrival at a cheap bunkering Port.
I then add the system quantity of the largest system using each grade of oil to the estimated consumption of that grade of oil from the Port we will be bunkering in, to the next cheap bunkering Port, and subtract the estimated ROB at the first Bunkering Port, check that we have sufficient Tank capacity to store this amouint of oil, and then place an order for this quantity, rounded up to minimise delivery charges.
Always order L.O. sampling kits with Bulk Oil orders as they are usually FOC ( Free Of Charge) then, but you are charged for them if you order them with oil in drums.
My reasoning is that you should always arrive at the Port where you plan to take LO with a reserve sufficient to replace a full oil charge in the largest stsem using each grade if you find that the oil has become contaminated or there is a major pipe blow out and you lose the entire contents of the system.
With Refrigeration oils, the consumption is usually so low that these should be ordered in the smallest possible containers. The oil is Hygroscopic, if the drum is not completely hermetically sealed the oil will absorb water from the atmosphere and start to emulsify. The oil will then cause severe problems in the reefer plant if it is used. The usual 20 or 25 litre drum available from the oil majors is far too large, most of the Refrigeration compressor makers sell approved Fridge oil as a spare part in one or two litre containers, there is much less waste if you order oil in this size container, that is all you need to do an oil change in a domestic reefer compressor and because they run cold the oil should be good for many years.
If interest rates are high and oil prices stable, or falling, it makes good financial sense to run with a minimum safe stock of oil as described above. However, if prices are rising and interest rates are low, it makes sense to fill up to the maximum while oil is cheap.
Again, I usually make up a spreedsheet listing every storage tank on the ship with their maximum capacities, so when the oil stock is checked each month you only have to enter the contents of each tank and the spreedsheet automatically calculates the total ROB of each grade and the maximum quantity that there is room to bunker and also if there is less onboard than the minimum ROB that has been decided for each grade.
It is always better to ask a stupid question than to do a stupid thing.