I don't do a lot of exterior varnish but I've been using the Manns range for interior varnish and they're great. The clear one's have not darkened the grain much on the area I've used them on.
Depending on the age you might want to strip it first and see what it looks like.
I'm going to strip the paint back door and want to put a clear coat to show the natural wood. Would that Cuprinol stuff do the job?
If I could use something that brings out the grain in the wood or at least doesn't make it loads darker, that would be great.