Should be fine to use. Not to worry. According to the MSDS
Storage/Stability
This product should be stored at room temperature and
protected from exposure to moisture. DMSO is a very
hygroscopic liquid. The purity of the material was
essentially unchanged per gas chromatographic
analysis. DMSO is thermally stable. It can be heated to
150°C for 24 hours with less than 0.1% loss in purity.3
When stored as indicated, DMSO has a shelf-life of two
years.
There's a science forum that talks about different people's experience with DMSO storage.
www.sciencemadness.org/talk/viewthread.php?tid=12675
Another question frequently asked is about the packaging for DMSO
It is usually shipped in HDPE bottles.
It is compatible with LDPE, HDPE, polypropylene,
PPCO polypropylene copolymer polymethylpentene,
nylon and teflon FEP.5