If u are running PWM as opposed to linear speed control the calcs are simpler.
There are a few parameters u must look at in the spec sheet.
RDSon = tells u the resistance of the device when it's fully 'on', of course RDSoff is infinite for our calcs => zero heat.
Thus u determine the voltage drop across the device using Vds=current x RDSon. U want this as small as possible.
For .012 ohm , Vds = .096V.
Power=heat= V*I = .096 * 8 = 0.768 Watt.
Thus the device generates 0.768 Watt under 100% duty.
50 % is 0.5 x .768 = 0.384 W
If you are running a relatively high frequency (>50Khz) then u will have to consider switching losses/heating. Also, that will require using a mosfet driver chip to reduce the switching time (Miller and Q effects)
I guess u know how to determine heat sinking requirements from that.? Note the Deg C/W of the Heat sink u want to use and cater for the loss of the efficiency due to the thermal paste.
Have a look here:
https://www.electro-tech-online.com/threads/heat-sink-sizing.119699/