300/4.9L vs. V8 factory recommended oil change intervals...

[1986F150Six]

The owner's manuals for my son's 1984 F150 and my 1986 F150 both state the recommended oil and filter change interval for the six cylinder engine to be 7500 miles. Under the same conditions, the V8 interval is 5000 miles. I wondered why?

I contacted a retired Ford Engine Developmental Engineer, who is quite successful drag racing with a 300 [cross flow head] engine. His "name" is The Frenchtown Flyer. Look him up on YouTube.

Here is what he had to say:

Originally Posted by The Frenchtown Flyer

That's a good question. I can think of two reasons. The six oil pan holds six quarts and the V8 holds 5. For the oil to get contaminated / depleted in a six quart fill it would take longer. Second reason - with a six you get about 3/4 as many firing events, so less combustion by products. Trust me, these things aren't arbitrarily decided. Only after extensive dyno and vehicle testing. Hope this helps.

[Gary Lewis]

That's interesting!  And I'll bet he's right as it makes good sense.

[Ford F834]

David, I have one of the little glove box “Ford Truck F150 thru F350 series Owner Guide” books from 1984. On the vehicle maintenance schedules it lists 3 months or 3,000 miles as the oil change interval for all engines.

In the specifications and capacities section it says the engine oil refill capacity (includes 1 quart for filter replacement) is 6 U.S. quarts for all gas engines as well (4.9, 5.0, 5.8 & 7.5).

I’m not sure why the discrepancies 🤷‍♂️

[Ford F834]

[1986F150Six]

Maybe I have egg on my face? I will confirm and report back. Thanks, Jonathan, for keeping things accurate.

[Ford F834]

I am going to play Devil’s advocate a little bit here just to see what people think. I certainly follow the logic on oil volume but I fail to see how the number of firing events would change anything. If oil contamination is primarily coming from un-burned hydrocarbons, intake particulates not trapped by the air filter, and metallic debris from internal parts friction, then (all else being equal) the big hitters are going to be the the volume of air/fuel burned per mile and the internal surface area where wear and deposits are happening. I am not an engineer by any means, but my rationale would be that contamination would correlate more strongly to displacement than the number of firing events. Thoughts?

[Ford F834]

David is correct, I was looking only at the section labeled “Severe Service”

My manual says all unleaded vehicles ~Except 2wd F150/4.9L is 12mo./7,500 miles

F150 2wd 4.9L is 12mo./10,000 miles

Leased vehicles except Canadian 5.0 and 5.8 is 12mo./5,000 miles

Canadian leaded 5.0 & 5.8 is 12mo./6,000 miles

[85lebaront2]

FWIW, many years ago I was looking at an SAE spec for oil sludge buildup test, the specified test engine was a 1968 Ford 289 so maybe the oil change interval was based on that test.

[NotEnoughTrucks]

Strange that the Canadian vehicles had a longer service interval. From what I've seen, many North American Ford trucks from this time period were built in Canada. We are in colder latitudes, so one would expect service may be a little more severe?

Maybe the same reasons that had us brew beer at 5%?

Anyways, recently I have heard that modern oils recommend longer service intervals. Saw an interesting Youtube video on the subject. Nice demonstration of viscosity at the end.

https://www.youtube.com/watch?v=tYkg0oDUXs8

Our fleet management at the railroad extended service intervals to 10,000 km. These were typically gas powered trucks and we routinely ran them to 500,000 km.

[Gary Lewis]

That's a good video.