Allison 1000/2000 External Spin on Filter & Internal 2nd Design 4wd (Deep Pan) White Filter Fits: 2010-On The spin on filter is mounted to the Outside of the Transmission Case just behind the bell housing. The internal filter is the one in your pan. This is a Suggested replacement on the Allison Spin on filter is a minimum of every 5000 miles. The internal filter should be changed and the pan dropped periodically to ensure that this filter has not trapped metal filings which are a tell tale sign of an internal problem, especially when towing a hefty load. Every 60 to 70K miles is a good time to consider doing this flush. Cheap insurance is better then an expensive rebuild that could have been prevented.