My 2009 nissan versa with the 4 speed auto trans holds 8.2 quarts total. When using the ATF drain plug, I can drain a little over 4 quarts. I have been doing a single drain and fill once a year (3-total) with Mobil 1. Should I switch to a drain and fill every 2 years as I will be replacing nearly half the synthetic mobil 1 ATF each year? My wife drives about 13-14k a year.