Before you change it, crawl underneath and see what size drain plugs you have. Many use a 14mm allen. Be sure to first remove the refill plug which is higher up. If you were to remove the drain plug and then discover that the refill plug would not loosen(it happens), you'd have a problem.

The viscosity you choose should match your local climate. 80W-90 is sufficient for most situations.
I do not know your units capacity, but many are between 1.0 and 1.5 qts. You drain and refill until it starts to come out the drop so in my opinion it's a waste of time wondering whether it holds 1.1 or 1.3