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Due to the frequent oil posts I hope I can be of some assistance.
First of I come from a family that owns a few gas stations here in brazil, my grandad is a former esso executive and i have an uncle that was an esso engineer, so I can say something about oils.

To start things of:
Oils in general change there characteristics with temperature, just leave some oil on a pan and heat it up and watch it change from a oily clear yellow to a dark glob. The first oil challenge was to create an oil that would maintain its characteristics with temperature change. A car starts in the morning cold, with all the cold oil in the pan, these first few moments are critical and can generate alot wear in the engine.
Thus, the choice of oil is hugely dependant on climate conditions. As oil viscosity grades varies with temperature, hence multi-viscosity oils (dah!).

The Numbers, explained:
5W30
10W40
25W40 ... and so on.. What do they mean??
In a nutshell: The first number actually refers to the cold temperature viscosity and the second to the hot temperature. This sets a threshold of ambient operating temperatures.
examples:
0W20 : from below -30C to 10C (residents of alaska and the north pole)
0W30 : from below -30C to 30C (russia, finland, iceland etc..)
0W40 : from below -30C to 40C (regions of alaska that have hot summers, or for people that live in the alps and drive down frequently)
5W30 : -30C to 30C
5W40 : -30C to 50C
10W30 : -18C to 40C
10W40 : -18C to 50C (most of the northern hemisphere would be fine right here)
15W50 : -10C to 50C
The more temperature spectrum they cover, the more they cost.


The argument: Mineral or Synthetic

Again, in a nutshell, it depends. While synthetics without a shadow of doubt offer more performance (due to less friction) and offers the advantage that can be changed less frequently, they are expensive. The argument of increased performance can be subjective due to the fact that these engines are getting on almost 20 years, so things like clearances, and especially valve stem seals tend to leak, generally on cold startups hence the smoke issues, the use of thicker oil can help remedy this.

My personal opinion favours mineral due to the facts above and that most of these engines require close to 8 liters for a change, and personally I prefer my engine to run new mineral over old synthetic any day.
Maintenance on these engines will happen, things like oil pan gasget, valve cover OR/AND its grommets, rear engine crankshaft seal etc WILL fail starting slow leaks, all of these will require an oil change to be added to the repair cost, again, here mineral will hurt less.

People with unlimited resources are welcome to use synthetic by all means. OH, I almost forgot, if you do track your car, oil change intervals will be halved, and please be nice to your engine and use proper racing oils like:
- Shell helix ultra racing 10W60
- Selenia Racing 10W60
- Valvoline Racing VR1 5W50
- Castrol Power1 10W50
Top Tip : install an oil cooler - cheaper than an engine, also change tranny and diff fluids (contact me if you need help on these)
(Hey racing costs money)

What to do:
In the U.K i would go for a brand name mineral 10w40 and change it every 3-4k miles (along with the filter obviously), depending on driving conditions, city driving requiring more frequent changes.


For the most of us that use the car as a DD, just go mineral and save the money towards more important stuff. There are much more important thing to do like: cooling, suspension (this alone if done right will almost cost the value of the car) and brakes (in that order)... 16 year old engines arent too picky on oil, just make sure it doesnt run out!


(if done right meaning: chassis and tower reinforcements, new control arms, bushings, springs, shocks, tie rods, engine and transmission mounts, xbrace etc.. going brand like lemforder, bilstein, PU bushings and so on..)
 
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