710 LE is specified for marine diesel and railroad diesel applications. The question then becomes ... How readily available is this product?
It is not essential that he use 20W40. Either 15W40 or 20W50 or even straight 30 weight are all acceptable substitutions are are far easier to find at Walmart or an auto parts store.
Walmart has been known to carry Shell's Rotella T in 15W40 for many years. The issue here is that the OP should be using motor oil and not some hydraulic fluid that would create problems for him. Choosing an easy to find motor oil such as Rotella and for a reasonable cost, makes perfect sense to me and to many owners of Colt, Case and Ingersoll GT's.