This discussion could go on forever, I ran Valvoline 20-50W Racing oil during break in with one quart of Marvel Mystery Oil (This stuff has been around forever). Then changed to synthetic, my engine leaked around the pan, the rear seal, and the remote oil filter adapter, and the valve covers.....maybe seeped is the correct word. Since I change oil every spring and only drive 1,000 - 1,500 miles a year I went back to Valvoline 20-50W Racing oil and my one quart of the Marvel Myster Oil, the leaking stopped. My dad was a big fan of Marvel Mystery oil, he had 50 years with the CAA, which later became the FAA, a Electro Mechanical Engineeer over Nav Aids. He used the Marvel Mystery oil as an additive in Diesel Fuel and engine oil in the diesel generators engines, back up systems for the Nav Aids if the power went out. Later on I became friends with Dale Pulde, NHRA Hall of Fame, Funny Car Driver, and Drag Racer, he told me they used Marvel Mystery Oil in the Funny Cars for years. So I use it in every engine I have broke in, Motorcycles, Diesels, ect. and then as aditive too.
Back to the subject, I see no reason to spend extra bucks for synthetic when I change every year, in my reasoning you must change every year. Even if the oil just sits in the engines there is chemical changes taking place, acids, bi-metal chemical changes, moisture, fuel, ect. So once a year the oil and filter get changed. Brian is correct the conventional diesel engine oils have changed and they are no longer what they use to be, high in zinc content however Valvoline Racing Oil is. I have a solid lifter engine....and that's the protection I need. I am sure there are other oils that have a good zinc content rating that would work good too. That's my take on the Cobra Oil.
One other note, I am an Amsoil fan and a dealer of their products, I use it in my Diesel Truck, Motorcycle, SUV, ATV, lawnmowers and their 2 cycle oil in chain saws, weed eaters, ect. very good stuff.