So this weekend I bought a new to me 91 f150 xlt lariat with the 5.0efi in it. I got it home and did a quick once over and then changed the oil and filter. I used 5w30 and a fram filter which I have since heard about the issues with and am planning on changing it tomorrow. My problem is that the truck makes great oil pressure while cold and it makes great oil pressure while over 1k rpms but the gauge shows no oil pressure when hot in park or in gear. My understanding is that the 302's achilles heel was low oil pressure but I also understand that sending units and gauges are also very common failures in these trucks. I hear nothing change when I have "No oil pressure" no knocking or anything which leads me to believe its sending unit or gauge? The gauge will never go above half either even when pulling hard like getting on the highway. Am i using the right weight of oil? Its cold now in Montana and I don't want to go too thick either. Thoughts and suggestions are greatly appreciated because I need the truck to get back and forth to work right now. Thanks in advance.

 

5 or 10W-30 is fine for cold weather. You will find out on here that the gauge is an idiot light with a needle. You need to put a mechanical test gauge in it to see what the pressure actually is.

 

Replace that Fram "orange can of death" oil filter and your low oil pressure problem may very well go away. It sounds crazy, but I did that once before I knew any better and experienced the same thing with my own truck. You are driving a Ford; use a proper Motorcraft FL1A oil filter. The owner's manual calls for 10W30 weight motor oil, so use it.

That is incorrect.

The 5.0/302 engine is a great motor and typically does not have a problem with low oil pressure.

 

Well yes and no. From my '89 owners manual




Unless I am way overdue for an oil change I have used Motorcraft oil and filters for several hundred thousand miles. Almost 300K on my '87 Bronco (302), 330K miles and counting on my Taurus (3.0L 24V) and 100K+ miles on a '94 Bronco (302)

 

A 1991 model is now 25 years old. That engine will probably need the slightly heavier oil.

Does it get below 0 degrees very often in Montana? If so, use 5W30 instead.

 

Yes it does....

 

Thanks for all the replies everyone. I replaced the filter oil and sending unit with a motorcraft filter, 10w40 castrol high mileage and a borg warner lifetime warranty sending unit(I know kinda heavy oil but I used to run it in my nissan all the time in the winter with no issue). The truck now makes oil pressure in park while hot but still doesn't in gear. Where would I get a mechanical sending unit and are they all standardized or are they motor specific? I still don't think its an issue of oil pressure simply because the motor would be knocking if I was making 0 oil pressure correct? Thanks for everyone chiming in on this. I hope we can get it figured out soon.

 

Partially incorrect. I have had two 302's die after losing oil pressure. Both were attributed to clogged pickup tubes.

 

JEGS Performance Products 41400: 2-1/16" Oil Pressure Gauge Black | JEGS

Or any parts store

 

And that would be maintenance, not design flaw.

 

Thank You.

 

Depends on how reliably it clogs and how long it takes for that to happen.

 

I've been responsible for a dozen Windsors over the course of my automotive life & never clogged one.

Your results may vary.

 

Do you have a tachometer in the truck? Kind of curious as to how many RPMs in gear while stoped its turning?