As a test, it might not be a bad idea. I would expect to see more of a difference after the second oil change without it, though- most oil has enough additives that another shouldn't be needed.
I don't remember- what's the reason for using STP?
I had a 1990 Silverado and in 2002, I was planning to go on a Grand Canyon rafting trip. I told my friend, who had worked for GM in V8 Powertrain Development, that I was seeing a puff of blue smoke on startup and he said that I should drive it out to PHX so we could change the oil seals at his place, where he has a lift. In the floor. With foot controls. When the valve covers came off, they were like new- no sludge at all. The previous owner was also a GM employee and Mobil 1 had been used since it was new and at the time, I estimated that it had about 150K on it, but couldn't be certain because the odometer had stopped working at 45K- knowing his driving distance to work gave a decent clue, though. I drove it until 2011 when I bought my cargo van and when I looked through the oil filler hole, it was clean and I could see the oil as it moved from the front of the head to the back.
The advice I got from the others who worked in the same area as my friend- keep it clean don't overheat it and change it before 5K miles. They said the brand wasn't as critical as keeping it clean and at normal operating temperature. We were taught the same when I went to MasterCraft boat service training. Look in the manual for the required oil additives and buy that- the SuperTech and Valvoline high mileage oils have everything your engine should need.
