dr bob

Low temp, pour point at 0c (the W rating), higher temp film strength at 100c is the upper rating. generally.


Thing to keep in mind is that the oil doesn't get thicker (higher viscosity index) when it gets hotter.

fabric

Right, but it's not as thin as if it were simply SAE10 oil. That's why I'm confused about Louie's post. I didn't think there was any type of viscosity continuity in the rating. i.e 5w30 acts like SAE20 at 70C, or something like that.

Louie928

It shouldn't. As Dr Bob said the 50 rating of the 5W50 means that the oil's viscosity at 100 C is equivalent to a SAE 50 class while the 5W means that the viscosity at below 0 C (Winter) is equivalent to an SAE viscosity class 5. This is the viscosity rating of the oil, not the lubrication ability. Don't get the two confused. The 928 owners manual only recommends multigrade SAE class 5W -X for temperatures below 0 C. It doesn't recommend 0W at any temperature.

In recent years it has become recommended by certain manufacturers to use multgrade oil with a wide temperature range and 5W or even 0W is not uncommon. One obvious reason is to get better fuel economy for the difficult to achieve EPA CAFE rating. A low voiscocity oil also help emissions when the engine is cold. Service departments have to stock fewer viscosities of oil when a wide viscosity range is employed. Some US manufactured engines, can't remember which right now, had poorly designed cam lobe/lifter lubrication and suffered from premature cam/lifter failure unless a low viscosity oil was used, such as 5W or 0W, because otherwise they starved for oil upon cold startup.

The 928 owners manual recommends 20W-50 for temperatures between about +15 F to above 100 F. That covers most of my 928 temperature range usage and that's what I use. The engine was designed for it. Lifter clatter on startup isn't normal. Others may have a different opinion regarding what oil viscosity to use based on other criteria and that's fine.

heinrich

I too have moved away from 15W-50 to 20W-50. Agreed with Louie.

algie928s4

Scary scary scary, "What the shop could have done wrong", really makes me shudder. A public thanks to Jim (Truescotsman) for all his help in the past.

nicobel

I'll keep you guys posted monday, i am working on the car this weekend either very early in the mornings or late in the afternoon (even with "lower" temperatures, it IS way too hot for me).
Anyways, car is going next thursday to Greg (P.Mtrwrks) for a full featured checkup. I guess is also time for a distributor/coil/plugs job.

Bill Ball

Just have Greg build you a stroker, and then you'll be fine.

Louie928

Hi H,
I know oil types and viscosity owners like to use is often a sensitive subject and people have different viewpoints. I try to stay out of that as much as possible when it doesn't make much difference. One question of using the lower viscocity oils, or wide range multigrade oils, is what's the downside of low viscocity? Dr. Bob mentioned he thought the upper number such as the 50 in 5W-50 was the equivalent film strength at 100C, and not viscosity. He's probably right about that. I don't know for sure. I'm too lazy right now to look it up.

The engine oiling system has a pump, pressure control valves, a pressure sensor, and filter, then goes on into the engine to do it's job. The engine bearings can be thought of as a series of leaks. Eventually, all the oil supplied leaks out. There is no return line such as with the fuel system. Along the way, it lubricates, keeps parts from rubbing together, and carries away heat. A low viscosity oil will flow more volume, will leak out more quickly and can be more effective in carrying away heat from the bearings. So far so good. The 928 oil pressure sensor is before the filter on the inlet end of the oiling system and only measuires the pressure at that point. Oil pressure isn't constant throughout the system. As oil goes through the channels and some leaks out along the way, the pressure becomes less and less the further you go. You may have 5+ bar pressure as read on the 928 pressure gauge at the pressure supply end of the system, but with viscosity too low, or oil too hot, not much pressure left at the end. The oil channel out of the filter is a finite size and will flow only a given volume at the supply pressure. Viscosity is one variable that deterines flow rate. Sharp corners in the channels, and there are many, further reduce flow as velocity and turbulence increase. The cylinder heads with camshafts, and lifters, are the last in line for oil. A sure sign you don't have enough oil pressure left for proper lubrication at the end of the oil system is noisy lifters with a warm engine. It would be more valuable to have the 928 oil pressure sensor at the end of the oil system so we'd have an indication of the lubrication pressure throughout the entire system. The other end of the spectrum is oil that is so viscous that it doesn't reach the far end of the system quickly, or flows so slowly to not extract much heat. It's a fine line between the two and the reason I prefer to stick with the factory recommendation regarding oil viscosity.

nicobel

Yeah Bill, That's my dream.
If only Greg offerd "easy low monthly payments..." the story would be much happier
but for now :