Can shock oil go off?

[Southwell]

Just finishing up my cars for the weekend and rebuilt my 2wd shocks, the rear have new losi 30wt oil, as in a brand new bottle and feel good. The fronts have been built with old oil, fresh but in an old bottle and feel crap, am i missing something here? When i built them with 35wt from a new bottle they feel awesome but with 40wt, which is prob about a year old their rubbish.