I don't believe in recommending a specific product but I would note the differences between a Sunblock and a Sunscreen.
Importance and Differences of Sunblock and Sunscreen
Sunblock-Although some believe that sunblock and sunscreen are both the same, they are not. Although they have similar properties and are both important in caring of the skin, sunblock is opaque and is stronger than sunscreen since it is able to block a majority of the UVA/UVB rays and radiation from the sun, thus not having to be reapplied several times a day. Titanium Dioxide and Zinc Oxide are two of the important ingredients in sunblock.
Sunscreen-is more transparent once applied to the skin and also has the ability to protect against UVA/UVB rays as well. Although the sunscreen's ingredients have the ability to break down at a faster rate once exposed to sunlight, and some of the radiation is able to penetrate to the skin. In order for sunscreen to be more effective you'll have to consistently reapply and use a higher spf.
There are two ingredients that I would look for in the above, Aminobenzoic Acid or Zinc Oxide.
UV-filter Other names Maximum concentration Permitted in these countries Results of safety testing
p-Aminobenzoic acid PABA 15% (5% EC-will be banned from sale to consumers from 8 October 2009) EC, USA, AUS Protects against skin tumors in mice. Shown to increase DNA defects, however, and is now less commonly used.
Zinc oxide 25% (US) 20% (AUS)(EC-25% provided particle size >100 nm) (Japan, No Limit)
EC,USA, AUS, JP Protects against skin tumors in mice[56
Finally, it is a myth, if you tan easily, that you are better protected from skin cancer. Depending on the rating, the higher the number the longer it will last, in a days tour, I would use a higher number and reapply every three hours if you use sunscreen. Be aware of cloudy days, when the uv rays could still cause skin damage. Applying the above may restrict the Vitamin D you get from the sun. If the above chemicals are not for you, there are several organic ingredients that may be effective too.