viscosity

[unklpat]

I'm aware that a lot of people use 20W50 in their U20 motors, but don't know why. Is it because of the low reading of the oil pressure gauge at hot idle? Is it because of the "death rattle" at startup? Is it because people believe thicker is better?
My motor is brand new, basically everything internally. I am using 10W30 Rotella as my break in oil, and my hot idle pressure is above zero on the gauge. I have zero noise at startup.
I used to use Amsoil 20W50 ZRod, but am thinking about using a thinner version. I know this topic has probably been beat to death, but what is correct? What was the factory fill, and why should we change that? Oils are much better than they used to be, so factory viscosity should be fine, no? Interested in any opinions or experiences. Pat

[Bwk2000]

My $0.02 Pat …

F1 race car engines are completely seized up when cold. The tolerances are so tight that nothing moves until the engine is pre-warmed by circulating hot coolant through it (think I read it was to about 175 F). In addition, the custom viscosity of the oil and additives used is specific to each car and a closely guarded secret. Each car will ONLY work with the specific oil made for that car due to the uber-tight tolerances unique to that individual engine. Needless to say, our cars are a bit more forgiving than that

To that end, throughout the years, I’ve seen many people use thicker oils (and/or additives) to increase oil pressure, reduce chattering or even slow down seal leaks on whatever they may be driving. Sure it will work for most mass produced engines but there are potential downsides as well. As you already know, thicker oil will reduce HP, labour the revving, and in colder climates, take longer to flow properly increasing engine wear. Also, too much oil pressure may damage seals, blow out oil plugs or damage bearings. Now, is this likely to happen with 20W-50? Short answer - Not at all. As long as we keep the oil pressure in the safe range and the cooling system in good order, pretty much any modern oil will work. That being said, I would always try and stay close to the factory viscosity recommendations no matter what I drive. They spent the big bucks on R&D so I wouldn’t have to experiment using my own money down the road

[redroadster68]

A datsun roadster mechanic and restoration specialist for over 35 yrs recommends for me to use Shell Rotella T4 15w40 in U20

[redroadster]

DYK -15 w , that's neg 15 weight oil is made , it's only for the artic antarctic regions .
Those thick oils can twist oil pump shafts
Most oiling parts simply hydroplane and the thickness isn't as important as the flow ,good flow . the valve stems and cyl walls ,and piston top ( cooled by oil squirting on its bottom)
may benefit from thicker oil on racing cond. Mostly the stems I think at 6-8 k sustained , and there are friction modifiers additives now that help that . Not sure on F1 , but most race eng. Are super loose .005 on rod bearings & crank and piston to cyl wall clearance xtra wide to rev quicker and use 20- 50 To add stability , but the whole block is modified , oil passages widend ....June thru August is the only time to try 20-50 in my book get a 5 gal bucket pump
Unkipat , try pumping 20w 50 vs 10-30

[Daryl Smith]

Both fuel and oil are much 'cleaner' and better today than when our cars were built. As previously stated, and I also believe, flow is more important than the thickness of the film barrier. Our cars were built around a 10-30W recommendation. I had previously considered even going to a 5-30W, but really don't see that it would be worthwhile. 20-50W seems a bit heavy for a street car, but, may(?) reduce seepage?

The U20 oil pump is a high volume (flow) pump to help keep the top end wet.

I will just be using a 10-30W, likely synthetic, that is made specifically for a car with flat tappet lifters (R16 based). Not sure how that equates to the pressures on the lobes of a U20 engine cam, but, probably a good place to start for any vintage engine.
*most modern oils are made for newer engines with roller lifters (less wiping action) or DOHC engines which usually see far less pressure on the cam lobes.

*IF you have installed an electronic ignition system, you are also more completely burning the fuel going into your engine and leaving less to 'wash the walls' and get into and contaminate the oil. The more completely your fuel burns, the longer your oil will stay clean.
I wouldn't worry about 15 or 20-50W oils twisting oil pump shafts as it's been used on these cars for as long as I've been around them (1982) without problems I've ever heard about...

[spyder]

And add zinc to keep the cam alive!

[Gregs672000]

Does a thicker oil create an oil film that provides a higher level of protection under higher pressures? I'm thinking specifically related to Daryl's comment on spring pressures in modern engines vs a U20, let alone one with a lot more lift and steeper ramps than stock. Cam wear is the only thing I'd be concerned about or have experienced myself. I run 20/50 Valvoline VR1 Racing with a NON-zinc additive that does not mess with the oil additive package ratios in the Valvoline per Bob the oil guy research (testing has shown that some OTC additives actually cause the oil to perform poorly by changing their package ratios) for this reason. My car is driven 90% of the time in summer temps (70-95 or so). I no longer have cam wear issues, but I can't say definitely that it's due to this or not, or whether I'd have an issue with a 10/30.

I recently added Motorkote but I haven't run it long enough to document any difference. Don't really expect any but I didn't see any downside.

[unklpat]

As I said, I'm running Amsoil ZRod, so no worries on wear, but don't think 20W50 is prudent or necessary. I was pleasantly surprised to see some pressure at hot idle and think tight bearing tolerances might have a little to do with it. I will most likely drop down to 10W40 and see what happens. Just curious what opinions are out there. Pat

[funkaholik]

Greg, what is the non-zinc additive that you are using? Dave Rebello had recommended I use Lucas TB Zinc Plus along with Valvoline VR1 oil. I'm also always curious to hear what's working for other people.

[Gregs672000]

Greg, what is the non-zinc additive that you are using? Dave Rebello had recommended I use Lucas TB Zinc Plus along with Valvoline VR1 oil. I'm also always curious to hear what's working for other people.

Hy-per Lube Zinc Replacement Additive, which is kind of a misleading name since it has NO zinc in it! It contains a polymer ester. Unfortunately it's not very popular and hy-per lube was bought out by Rislone, and their site no longer carries it. It's NOT a viscosity changer. I got a few bottles off Amazon.

[Daryl Smith]

Since Viscosity is a recurrent subject for these old engines I thought some of you might find this thread interesting.....

https://www.speed-talk.com/forum/viewtopic.php?t=67124

[JT68]

Climate has a lot to do with the choice. I’m sure up north a 10w30/40 is perfectly fine. In the boiling south and desert country, 20/50 runs like water. I’m running Mobil1 15w50 Hi Zinc now. The mobil1 products are generally outstanding -bearings and mating surfaces do extremely well. I’d trust Mobil1 way over anything made by Lucas.

[JT68]

No matter what oil you choose, it is important to warm up the engine before you beat on it-(get the oil up to temp). That has always been the case and somethings never change.

[redroadster]

It's hydroplaning that keeps your crank. Rods and Cam bearing from seizing...you know flow of a liquid thru a nearly trapped chamber so more is better not oil slipperyness
Try a pump that fits on a 5 gal barrel with
5 w 30 then 20w50 ....2 3 times the effort to pump at 60- 70 degrees ,now imagine you pumping it thru 1/4 -1/8 inch tubes
It's 20-50 real need ,if any ,is a heavy load 2 ton truck up hills etc.....in summer heat

[Daryl Smith]

"Climate has a lot to do with the choice. I’m sure up north a 10w30/40 is perfectly fine. In the boiling south and desert country, 20/50 runs like water. I’m running Mobil1 15w50 Hi Zinc now. The mobil1 products are generally outstanding -bearings and mating surfaces do extremely well..... "

Actually, ambient temperature is of little concern. It is the engine operating temperature, and of course the actual oil temperature which are important. Both of those are well above ambient in most places....I see no mention of climate in that thread...
You did read that a 5w-20 oil was being compared to a 20W-50? In a higher performance 454ci engine....

'Mobil 1 Racing' is mentioned there several times as a good oil, both lower and higher viscosities, as is 'Redline HP', 'High Performance Lubricants', 'Valvoline VR1', Amsoil, ....

The choice is yours, I am using a high ZZDP 10W-30 oil, maybe a 5W-30 (street use, occasional hooligan type driving)....a 15W/20W-40/50 obviously works, without shearing oil pump shafts, so why not? I posted this so that an informed choice can be made. My take from the posted thread is that yes the 20W-50 works, but it is likely costing you power, and possibly increasing your engine operating temp. YMMV.

Engine 'use' (racing, street) is more important than climate, and for the record, I don't know of anyone way up here in the north that runs their roadster year round, so temperature extremes (below freezing) aren't really in the mix....and before you go there, we do have desert country here also where temperatures exceed 40*C (104*F) on a regular basis, I am quite happy to not live there.

I will also say that I believe the oils available today are MUCH better than what was in use when these cars were built.