I have a 2005 Malibu and though the Oil Life Monitor still shows at 50% the dealer says I should have an oil change every 3 months/5k km. Thoughts on this? Will it cause problems on my extended warranty if I go with the monitor as opposed to the 3 month/5k km? Thanks for your help.

No, not when GM says change oil and filter when oil life tells you to or 1 year, which ever comes first. Dealer is looking to make a buck off you every 3K.

First , changing your oil is the one most important thing you can do for your car.
second, I have an Impala and I never wait till the oil monitor says change the oil. I usually change my oil 6 months 6k miles,
do you pop your hood at gas fill ups and check your coolant level and take the oil dip stick out ? does the oil look dirty, if it does get it changed.