Here in CO at mile high altitudes, the sun just tares up wood finishes on bird houses, furniture and ornaments. I have tried outdoor deck sealer (Thompson), varnish with UV protection, etc. The only one that worked for me the longest was porch and deck paint, not stain. I have gone to just leaving the cedar alone, like many fences are. I don't want to paint. I like the look of wood.

 Got any ideas that may help? Oil of some kind? Is there a really good sealer, that really works?

oil untinted paint base...

What stick said. Untinted oil paint (deep base, #4, or maybe #5) looks very much like varnish after it dries, clear with some amber cast and if you get the exterior paint it has UV protectors. I've had it last several years without needing refreshing, though it will eventually maintenance. Here's a link to the article that started it all, bear in mind it's a little dated. It may be hard to find oil based paint anywhere but a paint store and even then they are sometimes squirrelly about selling it without tint. The first time I bought it Sally Stockgirl at Lowes refused to sell it to me. This was back in the days when the mix counter had a fenced in area behind it to keep the good stuff. I had to go back later, sneak in and grab a can (Olympic #5 exterior oil base) and rush to the register to check out). Now some of the acrylic exterior enamels work also, I've seen (but not used) the SW A100 exterior base #4, it dries more clear than the oil based products even though it looks milky when applied. One nice thing about the exterior paint is that it's quite a bit cheaper and easier to use than the marine spar varnishes.

I've been using the deep tint base for years. Works good and lasts a long time.

The only thing is to keep it mixed up so it doesn't separate and will dry clear.

Or do not finish at all and let the cedar go to its natural gray.  Like most of the farmer fences are.

Another "Spar Urethane" failure, documented.   http://woodworking.formeremortals.net/2016/06/refinishing-patio-table/

Another view: (I'll have to admit, I could not make it all the way through this.   In typical Charles Neil fashion -- 10 minutes of video, 20 seconds of actual information.)

 

He's an extremely talented woodworker, but I can't say that his finishing advice is as good. But he's really a favorite over at Lumberjocks; that's the first place I had ever seen his name.

I had to fast forward to the end, but his conclusion seemed to be he's upset with System 3 Epoxy,  it was a pain to get on all the pieces and he's having problems stripping it off.   Better to go back and start over with new wood.

 

I have been less than impressed with Thompson's Water Seal, of which there seem to be multiple formulations (oil and water-based) but the oil based one might just be wax dissolved in thinner.