Puzzling cold start issues, am I on the right track here?

My sprinter does not like cold starts at all. Like anything below about 20-25 degree F it definitely wont start. Anyway I've started to try and figure out why. Cleaned the clear fuel line off and looked, no bubbles, took off the plastic top cap to the motor and super super clean so no injector leakage, the glow plug light has no problems and the starter seems to always move the motor a good speed despite it being cold.
So I believe the van has been sitting for a bit;
could it be a function of it just needing to be used more as I just got it? Also I have no block heater but I can get it to fire up by pointing a blow dryer at the fuel filter for an hour or more depending on the temp. Would a gummed up fuel filter cause this. I also thought about it could be old summer diesel still in the tank but I ran it below a quarter tank then filled it up to full and with a bottle of additive and ran it for another hour and still no help.
Anything I'm missing or is this a sure fire sign of bad filter?

Also sorry if this is a dumb question as im new to both sprinters and diesels but excited to learn more! Any motorcycle questions though...ask away!

THanks- Dylan

You may have issues with your Glow Plugs - I have at least one inoperable (I can start down to -15 F but my Sprinter is not happy about it). Any error codes?

Since you can get it to start by heating the fuel filter, I'd just replace the filter. They aren't expensive. How do the maintenance records look?

Dylan
Where are you located?,
what year model do you have?,
what oil grade and maker do you use?

I'm up in Stowe VT, I just put in rottella HD oil and Replaced the fuel filter did an oil and air filter change and she starts and runs like a top! Ready for this cross country trip! Thanks guys for all the help guys! Just gotta start with the obvious stuff!