I have been to the Guadalupe dunes several times. The best time is from February threw October. The dunes are closed during the other times of the year due to bird nesting. You need to get there before the ranger opens the gate at about 6:00am. Follow him in and go to the end of the road to a parking area. From the parking area head south/east out from the lot about 2-3 miles and you will find some spectacular dunes. Every time I have been here I have Never seen another soul out on the dunes! I usually go mid week. You may run into an occasional film shoot or wind. Be prepared for the wind in the afternoon. Start early and with the low light of winter you should have have great success. Now you know one of my secrets. Damn, why am I telling you this, guess I'm not so bad after all. I have made some of my best images here. Good luck.
Do you mean February through October like you wrote - or October through February?
Your post states the former - but you then speak about Winter shooting?
A LOT more from Guadalupe than from Oceano (far fewer 4x4s). I lived in Guadalupe for 5 years and my late father-in-law used to guard the gate to the dunes there in his early 80s (he never quite got the hang of retiring).
I found the dunes astonishingly hard to photograph. The canyons a few miles inland were easier but still difficult. Don't miss San Luis Obispo.
Your father-in-law must have turned me away a bunch of times because my car was 4 wheel drive even though was just going to take pictures or surf. I also had troubles when I was hired by Patagonia to document the Unocal diluent spill (petrolium thinner) in the Guadalupe Dunes that was covered up for years. I remember having to borrow a car to get out there.