As a creator of Gaskets and Drytops, here is my researched and tested answer.
It is best to trim the Gasket, with a sharp pair of seizers. There are ribs on the gasket; cut one rib at a time and test the sizing.
Whenever latex is stretched over time it will weaken, crack and degrade. You want the gasket to fit as tight as possible without stretching too much. Lots of stretching will significantly cut down your Gaskets life.
Sunscreen and bug repellent are terrible, for the durability of your gaskets, avoid contact as much as possible. Teflon based solutions, such as 303, do help.
For more information, check my site; http://bombergear.com/2009_2/product.php?id_product=39
Rick, Bomber Gear