Bob,
When I had my 948 rebuilt in Sacramento five years ago, I asked the mechanic/owner about these retro oil seals and didn't get a good recommendation from him. He had seen several types over the years and didn't think any of them were worth the money (for the amount of oil they kept from blowing by). He suggested I call Paul Asgeirsson at Morriservice in Portland (503-978-3998) and talk to him about his "fix" for the problem, which I did, and am pleasantly pleased to this day with the result. Essentially Paul's process involves rebuilding the worn area of the crescent shaped cap bolted to the block on top of the rear main bearing cap and of the cap itself, in order to close up the gap that has formed with the last fifty years of wear and tear. Anyway, the repair took a full day (two-three hours of actual work) due to the drying/hardening of the material used, but the result was (and still is) a leak free engine for around twenty bucks!
Hope this helps
Mark