Yaa..i have to admit that using the mineral oil will do the 'damage' to the FC as compared to fully synthetic oil. at least that was my personal experience. but u will save the initial cost by opting for mineral oil (saving RM100++) at the first place. I summarize my statement (as per my experience) as below :

Fully Synthetic Oil :
Initial cost : high
Running cost (FC) : lower
Drive feelings : usually the car will run smoother & more quiet, got better pick up (acceleration) & better engine protection (hopefully)
Periodical Maintenance : have to service the car only once for every 10k km

Mineral Oil :
Initial cost : low
Running cost (FC) : higher, as compared to when i use the Fully Syn oil
Drive feelings : usually will feel a bit sluggish as compared to when i use the Fully Syn oil
Periodical Maintenance : have to service the car twice for every 10k km (5k+5k). so at the end, the total service cost most likely will not be far away from the Fully Syn oil (not to say that u have to spend extra time, effort & of course some fuel to bring the car for service)

For me, it is not worth the saving (that the mineral oil will bring) when i put all the drive feelings, extra time, effort & the extra fuel in my considerations 

So, by stating that, u actually can judge for yourself the pros & cons of using both type of oil & which one u prefer

As an addition, the BK might also (slightly) contribute to the extra FC as it added extra weight to the car + extra surface area to be dragged by the air when the car moves. The drag equation are as follows :

[attachmentid=1682805]

where

    FD is the force of drag, which is by definition the force component in the direction of the flow velocity,
    ρ is the mass density of the fluid (air),
    u is the velocity of the object relative to the fluid (air),
    A is the reference area, and
    CD is the drag coefficient — a dimensionless constant, e.g. 0.25 to 0.45 for a car.

By using the equation, we will learn that the Drag Force (FD) are subjected to air density, velocity of the car relative to the air, the car reference (surface) area & the car's drag coefficient (which is 0.29 in value for Forte if i'm not mistaken). Since that we can do almost nothing to change the value of the air density & the drag coefficient, so only got two variables in our direct control which are the surface area of the car & the car's speed itself.

Note from the equation that more surface area (like installing BK) will contribute to more FD = more FC.
The most important thing & worth to note is that the FD increases with the square of velocity. So every km/h increase in speed will significantly contribute to the FD value thus the FC will also increases significantly. This explains why the FC is very high at a high speed coz the engine need to produce more power to overcome the high drag force. Am i sounds too technical?