What weight oil should I use for Oregon winters? [diesel]

I did some research and so far it's inconclusive. It gets a little below freezing rarely. I will be using synthetic oil, probably mobil 1, unless it's a bad idea because I have old gaskets. What weight oil should I use for the winter in Oregon?

Hey,

I would use the recommended 5w-30 for winter and 10w-30 for summer. The 5w is gonna be a little thinner which is going to transfer through your motor faster than the 10w which is thicker. Startup is the crucial point for lubrication and that's why they recommend a thinner oil in the winter.

Also, STAY AWAY FROM SYNTHETIC! On high mileage motors, all thats going to happen is it will run right by everything that needs to be lubed. Seals are worn, gaskets are worn and synthetic is known to leak on high mileage motors. PLUS, once you go synthetic you have to stay synthetic... $100 oil changes lol

Hope this helps.

Thanks so much, thermus, I'll do just that. Good lookin out, 'preciate it

No BAD advice for a diesel. Use diesel rated oil, 15w40 is the most common weight for diesels, but with higher mileage (300k+) and lower oil pressure you might want to consider 15w50. Stick with dinosaur oil, synthetic is pointless on diesels.

Thanks for your input. The information I post is all information that I had found online AND confirmed with my dad who is a mechanic and has been for over 30 years. He said that running a thicker oil wont hurt BUT is bad for startups. The reason is because the oil is thick and when cold it doesn't travel well through the oil jackets until it warms up. Cold starts are the MOST crucial time for lubrication.

Also, another important thing to note:

If your truck is a 5 speed, I would HIGHLY recommend bypassing your clutch safety switch. The reasoning why is that Mazda only used 1/2 a thrust bearing. This means that anytime you are starting the truck and pushing the clutch in, you are creating TONS of wear on that bearing and on the crank. I just rebuilt my motor and trust me when I say that this design can cause MAJOR HAVOC. Specially if you are gonna use a thicker oil in the winter.

Your advice above on the oil is good advice for gasoline engines, but can cause excessive wear on diesel engines. Diesel motor oil has additives in it specific for diesel use. One does have to be careful when putting in thicker oil in NEWER cars, you can actually do more damage to the bearing surfaces with the oil being too thick with the tighter tolerances. On older cars/trucks, not an issue as they are often worn down a bit and need the extra viscosity.

I do call shenanigans on the thrust bearing comment though, I've never heard or read such before. clutch in or out, automatic or manual, the amount of force on the thrust bearing is going to be the same when the car is started. Wear on the thrust bearing is most often caused by a poorly balanced rotating symptoms, often caused by swapping over to a lighter harmonic balancer.

I can understand why you would think this, however, the owner of the shop that I got my block and crank done at highly recommended bypassing the clutch safety switch. Do to these motors only using half a thrust bearing, they are more prone to wear and tear. He said that if the clutch is even over adjusted, it can harm that bearing and crank. Now, most people wouldn't ever notice a difference but for those of us that want to keep the vehicle for many years, it is an issue.

Not trying to call anyone a liar or anything, just going by what I heard from him and 2 other shops as well.

there may be something there that I don't understand about how pressing the clutch would cause additional stress to the thrust bearing, I'm not a master of physics so I won't claim such.

I do have one fear about the lack of a neutral safety switch, and that is accidentally leaving the car in gear while trying to start. It's easy to overlook when in a hurry, and can damage the engine, transmission, or someone walking in front of the truck. My 86 doesn't have one, and my brother tried to turn it on without pressing the clutch and while it was in first (i leave it in gear when parked.) Needless to say, he bought a new trash can and had to clean my bumper.

LMAO! That IS one down side. Yo uhave to make sure you are in neutral IF you are gonna start it without the clutch. Luckily, I was brought up since the age of 6 (started learning to drive stick then) to never park it or start it in gear.

The reason pushing the clutch messes with the thrust bearing is as follows:

Crank sits in thrust bearing> Flywheel, clutch and pressure plate are attached to the crank> When you push on the clutch you are pressing on 800lbs pressure plate springs which pushes that pressure into the crank. Over time, this can cause the crank to move.

This is not normally an issue because ALL other motors use a thrust bearing that goes all the way around the crankshaft. For whatever reason, Mazda's "F" block only uses a thrust bearing on the block site. :/