Ok before my other forum went down, i read someone said not to run 10w oil in a bronco II. Would anyone know why this would be, or if it is true?
Yeah, it's true. That's why i posted it. First, look at your owner's manual. For the 2.9, Ford recommends 5W-30.
The reason is simple on a lot of levels.
First, the engine was designed to run on that oil.
Why not run 10W, mainly because of wear, more than 90% of engine wear occurs at start up. The higher the initial viscosity, the longer it takes to get the oil moving, and the more wear results.
That's why no one uses straight weight oil anymore.
Multi-vis oils are really a misnomer. The lower number is related to cold temp flow, the higher number is related to operating temperature.
A 0W-30 will have the same viscosity, regardless of temp (up to a point, we could be talking about extremes here, but we don't live in Antarctica or deep space).
While a straight 30 wt will start out very viscous, then thin out as it gets warmer.
Since there were no 0W-30 oils offered in the late 80's, Ford recommended the best option.
If you're running 10W oil, you are wearing out the engine components faster.
If you want better protection, use 5W40.