Proper way of interpreting system load on a 4 core 8 thread processor

Not surely, but mostly on 1.00*n_cpu.

The load means the following: if there are multiple processes on a single-cpu system, they are running seemingly parallel. But it is not true. What practically happens: the kernel gives 1/100th second to a process, and then breaks its running with an interrupt. And gives the next 1/100th second to another process.

Practically the question, "which process should get our next 1/100th seconds interval?", will be decided by a complex heuristics. It is named as task scheduling.

Of course, processes which are blocked, for example they are waiting their data what they are reading from the disk, are exempt from this task scheduling.

What load says: how many processes are currently waiting their next 1/100th second time frame. Of course, it is a mean value. This is because you can see multiple numbers in a cat /proc/loadavg.

The situation in a multi-cpu system is a little bit complexer. There are multiple cpus, whose time frames can be given to multiple processes. That makes the task scheduling a little bit - but not too much - complexer. But the situation is the same.

The kernel are intelligent, it tries to share the system resources for the optimal efficiency, and it is in the near of that (there are minor optimization things, for example it is better if a process will be runned the longest possible time on the same cpu because of caching considerations, but they doesn't matter there). This is because if we have load 8, that means: there are actually 8 processes waiting for their next time slice. If we have 8 cpus, we can give these time slices to the cpus one-to-one, and thus our system will optimally used.

If you see a top, you can see that the number of the actual running processes is surprisingly low: they are the processes marked by R there. Even on a not really hardcore system is it often below 5. This is partially because the processes waiting their data from the disks or from the network are also suspended (marked with S in top). The load shows only the cpu usage.

There are tools to measure the disk load as well, imho they should be at least important as the cpu usage monitoring, but somehow it isn't so well known here in our professional sysadmin world.

Windows tools are often dividing the load with the actual number of the cpus. This causes some professional windows system administrator to use the system load in this divided-by-cpu sense. They haven't right and will be probably happier after you explain this to them.

Multicore CPUs are practically multiple CPUs on the same silicon chip. There is no difference.

In case of hyperthreaded CPUs there is an interesting side effect: loading a cpu makes its hyperthreaded pairs slower. But this happens on a deeper layer what the normal task scheduling handles, although it can (and should) influence the process-moving decisions of the scheduler.

But from our current viewpoint - what determines the system load - it doesn't matter as well.

As hyperthreading is not actually a 2nd core it will never take a core to 200% but it will take it beyond 100% for certain workloads.

So your maximum load is somewhere unknown between approx 4 and 6

(of course this can go up higher when overloaded because it actually counts runnable processes, particularly when they are waiting for IO)