Well, I've always toyed the idea of going full synthetic in the engine but always thought it wasnt worth it with a motor that has 115k miles on it. I've heard things like leaks appearing after going synthetic because the "gummed up" stuff breaks free and opens up leaks? What are the upsides and downsides of it? Convince me to do it and then tell me what weight I should go with. All year driving, cold months are coming up however!