That seems odd for a truck this new to suggest a 15W40 oil. I would have suspected 5W30 or 5W20. I don’t know but a lot of Toyotas have gone to pretty thin oil these days.
Honestly you just need to get that old oil out of the engine. 11 years even with barely any miles is TOO LONG. With the use of this truck, the oil type probably isn’t a huge deal. It just needs to be changed so that acids and moisture don’t corrode the engine from the inside out. You probably also want to flush the cooling system and do other maintenance as well. I could see the coolant being quite bad in a truck like this.
I am going to throw out two that I would suggest.
Mobil 1 0W40 European - You can get the 5 quart jugs at Wal-Mart for like $25. This is a great oil for that kind of money. I have always been told this is more like a 5w30 but was branded this way to meet the more stringent European oil standards while keeping in line with US emissions standards.
Rotella T6 5W40. This seems a little thick for some modern vehicles if you ask me but if your truck REALLY calls for 15W40, this is a very good oil for the money as well. This is really on the thick side of the 5W40 specs so be careful. I use it in all my outdoor power equipment and have no issues no matter how hot or cold it gets. The newer stuff with hydraulic lifters seems to really like it. The lifters used to clatter and then overpump on some neglected equipment I got. I would then have to wait for the lifters to bleed down so I could restart the equipment. This went away in short order once I changed to this oil.
And then I guess I will throw in a third… Whatever conventional oil this truck calls for. If it were to suggest 5W30 or 10W30, use the 5W30. Change it at least every 3 years and made sure you run it long enough to get the engine up to temp for a while.
My gut tells me there are all kinds of other things that will need attention as well. The coolant likely needs to be changed and the brake fluid is likely getting moisture in it from sitting as well. There are likely all kinds of little things that need to be done to this truck.