usually its 5W30 but this may differ depending on the engine, only really if its an RS model or XR2. though most motor shops (halfords definately) have a guide book in their store with details of which oil to use.
H4
13.5 cm
just about everything
My Mk3 fiesta did not.
diesel
10w 40 recomended, but for optimum performance use 5w 30
The rear speakers in the mk3 are 5 1/4 inch. If looking to install different speakers, an adaptor may be necessary.
http://www.fiestaguides.co.uk/content/interior.html
easy two plastic screws under seat
5w 30
5W30 oil
No, the floor pans are completely different. The MK3.5 will fit the MK4.