As stated oils aint oils.
Being around the automotive and heavy equipment for years there's a heap of things to take into consideration.
Whats the ambient tempatures the car is expected to operate in, Australia is quite unique in operating ranges.
Type of engine chains, most modern engines use oil pressure to provide tension on the timing chains.
How many k's has the engine done, more wear larger clearences means lower oil pressure, more likley to leak and the ability of oil getting past the rings and valve guides.
Use of a thicker oil would prolong engine life here and reduce emissions.
Petrol or Diesel, no way would l use either oils in the other without good reason.
Diesel oil is high detergent and cleans the engine internals, too well for petrol engines.
Petrol oil in diesels gums them up.
The only real difference in synthetic and mineral oil is where they come from in the refining process.
Always look for the SAE and check if thats whats specified for your engine.
Synthetics are a waste of time in pre ULP engines.
Do not use high zinc oils in ULP cars clogs up the cat.
Keep in mind the recomended Manufacture Brand is because they won a contract based on price and specification.
Always change the filter, not always specified at a service, neither is a air filter.
Now for my tricks and exceptions.
Change a petrol engine to diesel oil a few weeks before you want to do a engine rebuild, cleans up the internals and makes the dismantling easier, may start blowing heaps of smoke.
Older modern engines, use next higher weight oil.
With old diesels about to be traded, use heavy petrol oil, gums them up stops oil leaks and cuts down smoke.
Add one cup of kero before oil change and run for 15mins before changing oil.
Use a quility filter and know how many microns it is.
Oil sampling is a waste of time for the average car.
Now lets talk zinc.
Back in the 80's the oil companies were preparing for the introduction of ULP.
Zinc destroys catalytic convertors so the had cut it down but zinc improves the shear (wear) of the oil component boundry layer.
They didnt make this well known.
All of a sudden camshafts and lifters began to fail in new cars, many were repaired under warrenty.
New rebuilt engines were taking off cam lobes.
No one reilized why.
New procedures were developed to run in cams so they wouldnt fail as quickly.
5000rpm on a new 308 WTF. (even fluted lifters to get more oil on the cam)
Fast forward a couple of years a few of us went back to adding zinc to pre ULP cars and just run the engine in normaly without haveing a new engine screaming for 15mins.
l get mine from the US when required.
l have never taken a lobe off a engine l have built or installed.
Now what oil do l use.
Penrite HPR in the pre ULP cars l own, this has the highest zinc level of any modern oil.
HPR 50 in the Chevs and HPR60 in the Grp3
In the VZ's l use the Penrite synthetic 15 weight oil
Do them every 6 months regardless, get 20lt drums and dont pay much for it.