Polar fleece, at least the original and IMHO best "Polar Tech" brand is ranked and marketed in three weights 100 (lightweight), 200 (middleweight) and 300 (heavyweight). In addition, you need to be aware of styles that are intended to be worn as form-fitting under layers, and looser cut styles intended for layering. The form-fitting underlayer styles are usually made in 100 weight and not intended as an outer layer.
In both my May
Camino Frances treks (2013, 2014) I started with a grade 200 mid weight, long-sleeved fleece worn under a Gore Tex rain parka with pit zips. I continue to recommend this as the "standard grade" for outerwear.
However, on BOTH Caminos I was compelled to buy ANOTHER lightweight (100 level) form-fitting fleece to wear under the original looser fitting fleece. In BOTH cases it was not because it was cold, ambient temperature-wise. Brisk walking always warms one up. The reason was that I was soaking wet, either from perspiration or incessant rain. The reason is unimportant. That fact is that I needed the added layer to prevent getting too chilled.
The GOOD news is that polar fleece insulates very well even when wet. The BAD news is that you MUST keep your core temperature up. Remember, warm and wet is good. Cold and wet is very bad.
This is especially true if you stay in albergues and are exposed to inadequate or nonexistent heating, dampness, drafts, and a few dozen other folks, any one of who will likely have a respiratory infection spread by coughing or sneezing. I think you get the point.
On my first Camino, I bought an overly expensive body-hugging fleece jacket in Pamplona as I arrived totally chilled to the bone and pre-hypothermic at the end of April. I had walked in the constant rain and mud for two straight days, Roncesvalles to Pamplona. The added layer solved the core temperature problem. But, the North Face body layer fleece cost me € 149 - ouch!
There were no good choices on offer that day. The store was closing in 10 minutes on a Saturday evening and would be closed the next day. The selection was limited, and I felt like merde. So, it was buy the fleece or get bronchitis, or worse. That night I slept in my layered fleece, in my sleeping bag, under a woolen blanket in a heated hostal room in Pamplona. The next morning I felt much better...
On my second Camino this year, I made it as far as Sarria before the incessant rain and chilly morning temperatures compelled me to buy yet another form-fitting fleece layer. I had not brought the one I splurged on the previous year... The good news is that I was able to find one made by Trang World, in my size and in a color I liked at the Peregrinoteca shop on the corner, before the stairs in Sarria. It cost me € 50, on sale from € 99. I still have it. So, lessons learned and no harm done.
In 2015, discretion will be the better part of valor. The 2014 fleece will be stuffed in a compression bag and will go into the bottom of my rucksack, next to the 600 gm
Altus sleeping bag that I very rarely use...
I hope this helps...