My high mileage Toyota Tundra 4.7L V8 uses 5W-20. I recently acquired a case of Lucas full synthetic oil 20W-50 at $1/qt. Various people have advised me not to use it in the truck because of its viscosity. I have been thinking of flushing the engine with either kerosene or SeaFoam per instructions I have found on this site. What I would like to know is this: Can I use the 20W-50 oil with an additive for a short period (5-15 min) at idle without harming the engine? I know that engine flushing is controversial. I am leaning toward flushing with a mixture of oil and kerosene (ratio TBD.) But rather than buy some cheap 5W-30 oil, I would prefer to use this ultra cheap but thick oil instead. I can't imagine why it would be a problem to use 20W-50 for such as short period, but I could be wrong. Also, would it be risky to use 20W-50 instead of 5W-20 during the hot summer months? Your comments are appreciated. Please no anti-flush flaming.