Oil filter

update;

Due to new knowledge about the oil pump containing the pressure bypass and the filter providing drainback protection I will go straight to block like this;

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I spent a little time in Kragen today opening filter boxes to help decide which filter to use. I wanted the biggest one that would work and fit. The block is a 1989 which is same as a 1991. The chassis is a 81 so I thought I might worry about clearance but that is not an issue.
91 oil filter is a short squat PH16 prob same volume as a soda can
81 oil filter is a PH3675 which is about the size of a soda can
In 1980 and earlier, there was a PH11 which is HUGE and the size of a small peanut butter jar.
The 81 block used a small anti-drainback spring assembly captured under the filter thread adapter.
The 91 block has no such provision apparent and the cast aluminum adapter has nothing internally. The filter itself has a small anti-drainback O-ring built in.
So I removed the short threaded piece from the 81 block, removed the long threaded extension out of the 91 adapter and put the 81 piece in.


This allows me to use the early style filter which can be the small 3675, the fatter short PH25, or the fat and long PH11 which will give me almost double the filter capacity for now. Later, I will be installing a remote filter so the filter can be vertical and not leak when changed and not drain back at all. I will also install an oil cooler.