Hearts of Iron IVLooking at the system I think gameplay went up on all accounts here. I think the system feels more realistic and immersive when looking at how you need to build exact equipment, but with fuel costs calculated into production of equipment rather than when using them (like on ships). There are some suspension of disbelief issues here we want to work on in the future.
The only real issue with the HoI4 system is that it assumes an average lifetime-consumption of a product. Production costs the amount of oil that a tank is expected to consume during its lifetime for instance.
While that works out fine for the most part, there are some problematic exceptions to be considered.
Prominent example:
Historical Japan was crippled by the US oil embargo and wasn't able to sustain an offensive war without a constant oil supply.
When I play HoI4 Japan, I produce enough ships and planes before the embargo. Should I ever get embargoed and my access to oil is limited, I don't run into the same problems cause the system plays into my hands. I simply have to consider two things:
1. design units with 100%+ reliability
2. remember that you're fighting China and won't face any mentionable resistance in the air and sea.
==> Japan will hardly lose any planes and ships and thus vastly extends the assumed average life expectancy. You will soon reach the point where your planes and ships have already consumed the production cost of oil and from then on run for free. Without resistance they will run forever on the initial production cost.
A contrasting example:
Battle of Britain. The battle for air supremacy is a fight of attrition, whoever can produce more will win (with marginal technological modifiers). The average plane will live a day or two. The oil production cost though was calculated for a much longer time. This is where the system doesn't work either.