If your Dad oiled the action with engine oil it is likely that it is 'dieselling', and it may be that a particularly violent diesel detonation has ruined the spring. What happens is that the very high pressures in the air cylinder generates enough heat to ignite the vapourised oil and this causes an explosion which can put unbearable stress on the spring. I recommend that you buy a new standard spring and carefully follow the instructions in this thread:
http://www.airgunbbs.com/forums/show...74#post1884574
You can buy SM50 and Abbey LT2 grease from the sponsor's website. My .22 35E will shoot a less than 1" group at 45 yards in still air and it was made in 1978! You need to make sure the breech is correctly adjusted as slack jaws will mean large groups. These guns are superb and will last a lifetime, longer than your PCP I am sure.
Good luck.