6.5L Diesel Turbo is there 2 versions?

I was getting an oil filter for my 95 GMC 2500 4x4 6.5L TD and see a listing for a 6.5L Turbo Diesel vin F and a 6.5L Turbo Diesel (HO) vin S. whats the difference and which position of the vin letter are they talking about. I can't find anything anyplace i'm looking. I'm guessing one is the motor in most of the trucks and the other is the one in most tow trucks that they made with that motor. I don't really know and would like some more information. I found some info and was able to determin mine is the 6.5L TD HO, but what does that mean compared the the regular 6.5L TD.

F = L65, heavy duty (2500HD and higher), no EGR.

S = L56, light duty (2500LD and lower), EGR.

The 8th digit of the VIN indicates the engine model. Same now as then.

The biggest difference is 2x4 and 4x4. On the 2x you can run a long 1 qrt filter because it is vertical. I see you have the 4x4 so you need the shorter filter horizontaly over the front drive line.

thank you, thats useful info.