You can't use 15w40 in the Detroit 2 strokes, it causes them a lot of trouble. Port coking is a problem with multi-weights. A lot of abnormally accelerated wear will happen as well because of the loads a 2 stroke imposes, particularly during cold operation. Delo 100 is a good choice and still readily available from many places. There are other choices, use whatever you can find locally that is still a "good" oil. Problem with some of these older spec oils is some of the private labelers are just selling really poor quality oils because demand is so low these days.
I went through this with a company I worked for. They ran a large fleet of 6v92TA's. They were getting around 30k miles on a rebuild (severe duty). Part of the problem was the parts we were getting from DD, but a lot of our problem went away when I forced them to go back to a straight 40wt oil of good quality.
As for the engine itself if it's a 1980's GMC you could have a few different beasts. If it's a Brigadier then it probably has an 8v53 in it. If it's a General it's most likely an 8v92 but may still have a 71 in it depending on what year and how it was ordered.
They were good engines, but they did have their issues. Very durable and capable of quite a bit of power. I have an 8v92 that is around 2800HP and lasts quite a while for a race engine.
I went through this with a company I worked for. They ran a large fleet of 6v92TA's. They were getting around 30k miles on a rebuild (severe duty). Part of the problem was the parts we were getting from DD, but a lot of our problem went away when I forced them to go back to a straight 40wt oil of good quality.
As for the engine itself if it's a 1980's GMC you could have a few different beasts. If it's a Brigadier then it probably has an 8v53 in it. If it's a General it's most likely an 8v92 but may still have a 71 in it depending on what year and how it was ordered.
They were good engines, but they did have their issues. Very durable and capable of quite a bit of power. I have an 8v92 that is around 2800HP and lasts quite a while for a race engine.