I fixed a leaking sliding glass door today

well, mostly anyway.

Last time I used plastic tape and it left some marks.
Today I squeezed some 3/8" foam caulking backer rod (frost king) in the spaces around the sliding door (the stationary part seemed okay). It fit well around the top and sides, not so on the bottom, but we already had a draft stopper there.

You really can feel the leaks when it's cold outside.

I'm trying to follow what you did as I have a moderately leaky sliding glass door. What 'spaces around the sliding door' do you mean? All I'm picturing are the areas where the foam backing would get kicked out when the door was slid open.

This pre-supposes the door not being opened, sorry.

It works better than the clear tape, I think.

Well, naturally I wait 'til it gets to 3*F before I do this again, but it was really cold by the door today.
The foam caulk backer makes a big difference.
I put 1/2" diameter Frost King Poly Foam Caulk Saver, Part C22, in the vertical seam near the lock.
I put 3/8" diameter of the stuff, Part C21, in the other seams.
The stuff is available in a lot of places.

It made an immediate noticible difference.
Those seals on the door must be crap, or it's just a nature of the beast.

I lived with a crappy old aluminum framed slider for years. Finaly a builder freind gave me a nice vinal over wood slider. The builder and my freind Itinerant Bob installed it. Wonderful! It doesn't leak, nor does it conduct heat (or cold) through the frame like the old one did. Aluminum is the second best heat conductor of the metals , only copper is better (or worse), a poor choice indeed for doors and windows. The new door was taller, I had to make a steel header reinforcement, so we could cut two inches from the door header. At the time I wondered whether it was worth all the effort. In the end, it was well worth it. One of the single best thermal efficiency improvements I have made to this house.

Thing is, mine is a late model Pella slider.

Wow, thats messed up. My "new" doors were old when I put them in and that was a few years ago.

Yes Jags, I think I could've used a section of smaller diameter backer rod, because what I had wouldn't go in the crack and I taped it down with some of that clear tape meant for the purpose.
Last season, I rolled them up and zipped tied them together. The baggie idea is good. Of course there's always the question of "where did I put that darn thing" when the time comes. I put it on the storm door glass in the basement, the theory being I'd do them both at the same time. Good intentions, I suppose.

when buying a new slider doors get Anderson brand.. works well for me i have lived at a few places and they were not Anderson and now my new house they are awesome no cold air leaking in...

Good point, but technically incorrect. I don't see too many gold patio doors though, so we can let is slide just this once.

Talking about heat not electrons.

The thermal conductivity of gold and silver are both greater than aluminum. Silver is slightly higher than copper, and gold is between aluminum and copper. Aluminum is by far the best heat conductor for the money, many times better than carbon steel.

We have a 20+ year old, dual-glazed, 8ft. Andersen slider that seals very well. We give it a lot of use as we let the dogs in and out that way.

I just wish we could find a decent treatment for insulating it a night. As big as it is a fairly large convective airflow occurs and you can feel the cold air washing down off it. We installed some vertical louvered blinds and they actually work well at night...when you can operate them. The brand we got stinks as far as operation.