1990 350sdl

[jtbird59]

I have a 1982 300sd turbo which I love. Runs great. I recently came across a beautiful 1990 350 SDL (I know, the infamous rod bender) and bought it. I went against my better judgement. It was just to nice to walk away from. It has 139,750 miles on it and I just drove it 300 miles, mostly highway and it seems to run perfect.
My questions to the group are:
1. are there any of these engines that do last
2. are there any know preventative measures one can take to increase the longevity
3. is it correct that if driven mostly highway miles rather than city it may last longer
4. how long can I expect this engine to last given gentle treatment and regular oil (Amsoil marine diesel 15-40) changes
5. what will be a good replacement engine. Will the 5 cylinder bolt up to the trans and everything else, Or is there a factory replacement for this 350

The body, interior, chrome and electrical are all like new. This car rides beautifully, not a squeak, rattle, wind noise or shimmy. I'm willing to take the financial hit for a replacement engine if and when the time comes. This is a car I would like to keep for the next 10 years or longer. I do keep some of my cars for a long time. Have had (and still have) my Grand Cherokee since new in 1993 and my 1959 Tbird convertible since 1972.

Best regards,
squarebird59