Supply Path Value (SPV) / 5 = % of oil/res. lost. (rounded up)
To obtain the SPV see p. 188 of the manual:
Example: Base 1 and Base 2 are connected by 5 railroad hexes. The SPV is 10, and 2% of all oil/res. points moving from base 1 to base 2 will be lost to shrinkage.When tracing a supply path for movement of ground units or overland movement of supplies, a
supply value is generated. This is determined by tracing a path from the base transferring
supplies or the ground unit moving to the destination. This supply value determines whether the
move is legal, and how much of the supplies are used up during the move. The supply value of a
move is calculated by subtracting the following from 100 (900 if a ground unit move):
- 2 for each hex moved along a rail/highway
- 5 for each hex moved along a road
- 25 for each hex moved along a trail
- 50 for each hex moved cross country
However, every turn there's a chance that oil/res. points will not move at all, and this chance increases with SPV. A SPV below 20 almost never causes oil/res. movement to fail, but with SPVs larger than 50 oil/res. movement will fail frequently. A SPV larger than 70 is a guarantee that oil/res. will not appear in sufficent numbers at a base that needs raw materiel. See post #2 for screenshots of some test runs with different SPVs.
Shrinkage during port-to-port movement: A different set of tests showed that adjacent bases separated by ocean hexsides must have at least level 3 ports for port-to-port movement to occur. Shrinkage during port-to-port movement is always 20% of the oil/res. involved. Port sizes larger than 3 have no effect on the amount of shrinkage; it will be 20% between level 9 ports, too.
How many oil/res. will move from base to base? The AI will attempt to draw oil/res. from other bases as soon as the stockpiles drop below 2xHI. The base will then attempt to draw enough oil/res. to keep the HI running for 16 days +25, but the AI doesn't take shrinkage into account, so fewer oil/res. points will actually arrive. The amount of oil/res. still present at the base is multiplied by 4 and substracted from this number.
Example 1: A base with 1000 HI and 0 oil/res. will attempt to draw 16x1000+25 = 16,025 oil/res.
Example 2: A base with 1000 HI and 1500 oil/res. will attempt to draw 16x1000-(4x1500)+25 = 10,025 oil/res.
I don't know why raw materiel already present is multiplied by 4, nor where the 25 comes from.
Bases with less than 1000 oil/res. will never give them away. However, as soon as the stockpiles are above 1000, all oil/res. points can move to other bases.
Example: A base creating 100 res. points every turn will give them away only every 11th turn, but then all res. points can move to other bases.