Join Date: Apr 1999
Location: Midland County,TX, USA
Feedback Score: 0 reviews
If you mean pour in 4 quarts of synthetic along with 11 quarts of dino when you change the oil, it won't hurt a thing. It probably won't help much either, but it shouldn't hurt anything. You'd wind up with a DIY blend.
Any time you change oil, you're going to change only 15 quarts of the total 16 quarts in the system. There's about a quart in the high pressure oil reservoir and oil passages in the engine that won't drain. So when you get done, you'll have one quart of old oil, 4 quarts of 5w40 syn, and 11 quarts of dino 15w40. That should work fine as long as you're not expecting the wintertime starting ease of all 5w40 syn.
But the next oil change I'd send in an oil sample to Blackstone for analysis. Tell they your blend of oil, and ask what the results are, good or bad, of using that blend. I'll bet they say the blend was fine and dandy.
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My Sierra Blanca in the sig pic was a great pickup for 11.5 years. I sold it a coupla years ago. I drove a hand-me-down 2003 F-150 SuperCrew 4.6L 2V for a while, but it was unacceptable for towing more than a rowboat. Replacement is a 2012 F-150 EcoBoost SuperCrew Lariat that tows my 5,000-pound TT like a dream.