Can i use 5W20 oil in my 1991, 4.9l f-150? Will it cause damage?


Asked by Apr 16, 2015 at 06:36 PM about the 1991 Ford F-150 XLT Lariat LB

Question type: Maintenance & Repair

I have a 1991 f-150 long bed, which I rarely drive...trips to the dump, etc.)
Without paying much attention, I changed  my oil.  I had used 10W40 in it last time
I had a 5 quart jug of Valvoline Max-Life 5W20 in my toolbox for a long time, and didn't think to check the I'm wondering if I need to change it AGAIN!

3 Answers

Tom Demyan

Since you rarely drive it, I would think it would be fine, when you have to add or change it next time just add 10W-40 to keep the level up.

1 out of 1 people think this is helpful.

The two numbers associated with the oils refers to their viscosity. (the thickness of the oil) The lower the number the less thick it is. The first number (5W) refers to it's properties in cold weather 5 is less thick than 10, so in warm weather you should be ok. The second number refers to the viscosity of the oil when hot, engine running. 20 is thinner than 40 and flows much easier. You engine is much older and no doubt has some wear on it. You may see a little smoke from the tailpipe using 5W20 as thinner oil may slip past the rings and cylinder wall and be turned, and a slightly noticeable reduction in power. (loss of compression) with thinner oils. 5W20 oils were only developed within the last 5 years or so and are called energy conserving oils. Engine components on newer engines are machined to much closer tolerances, requiring thinner oi,l to increase MPG (cafe figures)


correction "will be burned."

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