I know I've been posted over since writing but I'll post it anyway.
Its funny you posted this when you did. I've just come up from the cellar after cleaning my gear from Mondays trip. I've used a host of carabiners but these days I stick to Petzl, DMM a few CT and the odd Wild Country. They are all good after many years of use but you do need to clean them regularly.
As for lubricant, WD40 technically isn't one, its a water displacer. Using WD40 will attract dust and dirt and make the gates stick.
3 In 1 lubricating oil is the best I've found. You only need a little drip onto the spring and a drip or two into the threads on the sleeve, if its a screw gate. Washing up liquid works quite well too.
Petzl carabiners have an indefinite life expectancy so its down to a visual and tactile examination to determine whether they should be scrapped. The user instructions should show you how to do this. Have a look on Petzl's website, there are some videos showing how to examine all the equipment that we might use underground. Some manufacturers may recommend a 5 or 10 year life expectancy but there is no need. The issue of life expectancy of carabiners was the topic of an IRATA workshop many years ago. Denny Moorhouse demonstrated there was no need to apply a life expectancy to carabiners through a series of destructive tests. He had a stash of old carabiners from when he first set up Clog, which later became DMM. Denny parted company with DMM and set up ISC (International Safety Components), he has since retired.
I generally buy myself 3 or 4 new screw gate carabiners every couple of years for my personal SRT kit and then demote the previous ones for rigging. If you avoid dropping them, never leave them in the bottom of a damp tackle bag and clean them regularly they should last for decades.
Pete K's point about the type/quality of the springs is important. I've never had any issues with springs from Petzl or DMM but some of the cheeper steel carabiners are well known for the springs coming loose. They don't fall out, they just wobble about and no longer work.
Petzl recommend scrapping textile and predominantly plastic products after 10 years from date of manufacture. The date of manufacture is marked on all Petzl products. Its a UV issue why this 10 years is applied so not that relevant to us. A helmet should last a lot longer than this if it is looked after and I would expect a harness to be pretty worn out after 10 years. I usually replace my harness every 18 months to 2 years and I replace my cow's tails every 6 months or so, depending on what type of caving I've been doing.
Hope that helps.
Mark