I changed my gearbox oil today but afterwards I noticed that the bottle said 75w-90 Semi-synthetic, this got me curious so I rang ford to double check the spec and they said that I should use 75w-90 fully synthetic.
Will I be doing any damage is I continued driving with semi-synthetic?
Car is a ford fiesta 1.25l petrol
As long as the oil has the correct weight (75w-90) and is SAE approved, there should not be damage to the manual transmission.
The chemistry affects the flow characteristics with age and temperature.
Fully synthetic oils maintain their viscosity over a wider temperature and for a longer period of time and is recommended when cost is not a concern.
For cost concerns, semi-synthetics and conventional can be used, but require inspections over a "shorter" period of time/mileage than fully synthetic oils.
This answer will change drastically if it was not a manual transmission (instead, say an automatic transmission where the fluid not only provides lubrication but affects the hydraulic flow characteristics of the transmission completely. Honda automatic transmissions are well known to have this requirement of working only with OEM fluid)
