Steve
there are still weights of oil even in the Rotella line, so with an "older" engine use one towards the higher end of your weather spec...some of the thinner oils may "slip" by and cause some smoking an older style non-syn might not..
My 1991 4jh2 with 2300 hours has no issues with the change to Rotella..
definitely change the oil and filter if you switch to the rotella, or other synthetic from a conventional. Have some good rags/absorbent pads (some use pet accident pads - cheaper) as the synthetics are a tad tougher to clean up or remove from surfaces.
I have gotten good service for the last 12-15 years using rotella in all my engines, cars, boats, etc...still use Harley Davidson heavy oil in that though. Other wise no issues at all..
best of luck