Synthetic paint problem

hi bought some synthetic paint after reading how easy it is to roller on and get a good finish on my Landy.......
Well mine looks terrible:-(
The paint is really thick, very hard to stir. does it just need thinning down? I've not read anything on anyone else having to thin it down for rolling/ brushing it on. I'm using a good roller for gloss paint. Could it just be because it's bloody winter and about 5c in the garage?
Thanks

I've never rolled car paint only sprayed it but thick paint and cold = bad finish when spraying .
Can't see things being much different when rolling tbh.

Anyone know if it does need thinning, is it a particular thinners I need?