I have a few questions.
1) How hot is it where you live? I've never seen or heard of grease melting in a grease gun in my climate.
2) What level of Hell do you live in?
3) If the grease is liquifying in your grease gun, wouldn't it also be liquifying in the greased places on your car?
4) How well can liquified grease lubricate your car?
5) How well can the car's rubber boots retain liquified grease? Do the boots leak/drip grease?
6) Is there a better grease you could get that won't liquify?