My girlfriends car is a 2009 Pontiac Vibe, she actually has the same kind of engine I have in my 2005 Toyota Camry. Her engine takes 0-20, whereas mine is 5-30. Is it plausible to use at least 5-20 during winter time? And considering we have the same 2.4L then why does her car require a dramatically different weight oil?
I recommend looking for the updated oil specs for each of your cars. From the course of the late 90’s to the early 2010’s Toyota went from 5w-30, 5w-20, and 0w-20. Some years and engine overlap where you can use all 3 types. Some only 2 types. And some only 1 type. It all varies depending your model and year.
Using 5W-20 in her engine in the winter time isn't really something I'd suggest if 0W-20 is recommended by the manufacturer. The 5W-20 will take the oil pump slightly longer to pump through the engine when it's cold and will eventually cause more wear.
Sometimes they change bearing, journal and cam designs, etc. on what is otherwise the same engine as they find out there may be minor flaws in what they built. When they make those modifications, sometimes they change the recommended engine oil.
