Delo would be a good choice . It works very well in older diesels and handles high heat well without burning off.
While I am one of the worlds biggest fans of Amsoil ( been using and selling the stuff for over 35 years and have installed more bypass systems than most folks have probably ever seen

I would not recommend dropping it right in. You would most likely have unhappy results. I would strongly recommend runing a couple treatments of
Auto Rx through the thing while running Delo first to get the insides of your engine completely cleaned out , to where your rings are fully clean and freed up and your seals are all clean and conditioned and not leaking, THEN your engine would be ready to convert to Amsoil.
The 15w-40 diesel and marine has a 25k mile change interval in our engines when used in normal service, 15k severe service, this is WITHOUT a bypass filter. When used with the PSD size Amsoil full flow filter ( which , by the way, filters down finer than many so called bypass filters) you can leave the same filter on teh whole time as that filter is rated for 25k service( and is the only filter made built to run that long, do not try it with any other or they WILL clog and go into internal bypass, i.e. run unfiltered oil through your engine )
With testing , you can run longer, and with bypass filtration, the sky is the limit. I run the 15w-40 diesel and marine with bypass filtration and havent changed my oil in over 80k miles . Last test was at around 70k and everything was fine. My engine consumes oil at the rate of about 2 qts every 5k miles or roughly the same as an engine where someone doesnt ever have to add any between 5000 mile changes, it just so happens that at that point, I keep going so I cant be one of those liars who says my engine *never* uses any oil, because I must top off and continue on for the next 5k miles