hi guys got in p38 year 2000 today and put it in reverse did not like it will go but very slowly felt like it was fighting its self any ideas mate told me to pull gear box dip stick and check levels but cant see one

hi guys iv had electrical problem with motor warning lights coming on saying doors open tail gate open gearbox over heating got that fixed but gearbox turns out was not electrical that still coming on, no fault code showing when you plug computer in garage that did work think its the oil in gear box is problem but they just do electrics.
can you just change the oils in them does any one know because first local place i rang said new gearbox and i love the car but dont think i can afford new gearbox.