I have a 90 model Ford f-150 pickup truck with a 302 engine
Can I use diesel fuel mixed with the old oil to flush out my engine before an oil change?
No, you can’t. Just use good quality oil and change it often, every 5000 miles, and drive on.
My dad used to do that w/his 60’s F100 v8. Not recommending it, just saying. I don’t think it helped or hurt in that case. On a modern engine like yours I’d worry about how the internal engine gaskets will hold up to something they may not have been designed to interact with.
It is your truck so you can do anything you want to it. It is a really bad idea though, who knows how much crud yo may flush into places where it can do damage. Why do you want to, is the engine full of sludge?
No flush should be needed if the oil change regime was maintained properly. On “Oil Abused” engines there can be all sorts of crud adhering to the internals of the engine. When you use a solvent to dissolve these deposits you can really open a can of worms. If the engine is running fine then skip this idea and just keep changing the oil or change it a little early… 3K miles used to be the normal schedule and today would be considered a bit “early”… I would just stick to 3K miles oil change interval. If you currently have no oil related problems you will be just fine by doing this, but skip the flush if there are no issues.