Yes, leave old filter in while doing this. I am not exactly sure of how much fluid.
Either way will work. Just remember, if you flush out the fluid completely then chances are that you are also getting the fluid that mainly just sits there at the bottom, which accumulates grime over time. This means that while the fluid travels through the system, so does the grime. Unfortunately the grime doesn't all leave, it sits in parts of the system that can damage your vehicle in the long run.
I always recommend using the dealer's specified fluids. I have used Castrol in the past and never had issues with it. Same for other, so it is your call. I believe the only difference in pricing is that the cheaper one will burn up a little faster than the other.
Sunday, December 21st, 2008 AT 12:16 PM