Don't save your sunblock from one summer to the next. Buy the size that will be adequate for the time you need it and then discard. Sunblock that is over a year old has started to deteriorate so the benefits will be diminished. When purchasing sunscreen make sure you check the active ingredients...if Zinc Oxide or Titanium Dioxide are not listed you wont get the maximum benefits. Try Skin Rx Clinics Advanced Protection SPF 30 Sunblock with Zinc Oxide. This physical sunblock provides full-spectrum protection and has vitamins & anti-oxidants.
There are two kinds of sun protection: Physical & Chemical.
Physical sunblock does not have the ability to be absorbed into the skin and as a result sits on the surface and reflects the damaging rays away from the skin. Physical sunscreens will always have Zinc and/or Titanium Dioxide.
Chemical sunblock has the ability to be absorbed into the skin and as a result must be reapplyed to keep its effectivness.
Hope this helps!