Allison 1000/2000 External Spin on Filter & Internal 2nd Design 2wd or 4wd (Shallow Pan) Filter Fits: Late 2006-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. The Suggested replacement on the Allison Spin on filters 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.