When i went to toyota to get a key made, they said that the transponder has to be programmed to work with the ECU, cost something like £65 to get it done.
"n/a got me 300 miles with 25 quid" really? that must of been ages ago when fuel was cheaper starbo with £20 vpower expect around 95-120 miles. thats with todays prices
have you tried to heat it up a bit? plenty of WD40 will help. some dont come off easy as others so you will need to give it few harsh hits evenly all the way round. hit the exhaust side and keep checking progress as you dont want to damage any blades.
oil cooler probably on adds around 0.5 ltr so you dont have much to worry about. just fill it to Max, then idle or drive for a bit and top up whats needed.