Awesome! Please post pictures!
Where in CA are you? That is an important element to it...
See below: This is related to MB diesels, but stands true for the older MB gas engines too...
An older and newer version of the chart that is easier to see. The newer one is interesting, as it shows how he change in specs and standards broadens the range of specced oil.
In reality at temperature under load, were considerate of the HT/HS viscosity as a measure of protection. But to get that, we need to be aware of the basics.
The differences between different viscosity oils, conventional and syn, are large - even at the same labeled viscosity.
Let's start with the 'w' portion, like 0w- 5w- and 10w-
These correspond to the Low Temperature (°C) Cranking Viscosity, in units of maximum cP (centipoise), tested by the ASTM D5293 method. The specs are:
0w- => 6200 cP max at -35
5w- => 6600 cP max at -30
10w- => 7000 cP max at -25
And so on.
So, what do those mean? They are the definitions of the highest the viscosity that an oil can be at some test temperature. Some really good oils can meet more than one, for example schaeffer's 15w40 diesel oil actually can test out to be a 10w-40, due to its performance at -25C, similarly, Amsoil 'acd' qualifies as a straight SAE30 weight oil, but its performance at low temperatures also qualifies it to to be a 10w-30 as well. Viscosity/temperature curves are not linear, relating to the coiling/uncoiling of the viscosity index improvers (polymers spoken about earlier) as well as the molecules of the oil basestock itself.
Now, the higher number:
These get a little bit trickier. Two 30 weight oils can span a fairly large range, and still qualify as a 30wt. In general:
Xw-20 is 6.6-9.9 cSt at 100C
Xw-30 is 10-13 cSt at 100C
Xw-40 is 13-16.5 cSt at 100C
And so on. The ranges are not identical.
So, you can have a very thin 30 weight, like Mobil 1, which is only 10 cSt, or a very heavy 30 weight oil, like the made in germany castrol syntec, which is about 12.1 cSt.
Now, there are also ratings. API, ILSAC and ACEA. What is interesting is the newer viscosity temperature chart, which broadened the range for 10w-30, for example, if it met the CCMC G5 spec. Why? Because that was actually the precursor to the euro (ACEA) oil specs. What is interesting is that this spec was the precursor to the A3/B3 spec, which is for higher stress conditions. You can have an A1/B1 spec oil in any viscosity you like, or an A3/B3 oil in the exact same viscosity. The difference? Robustness. The A3/B3 (CCMC G5) will have a higher high Temp/high shear viscosity, which means that under severe temperature and load conditions it will retain robustness and full protection, while also being lower viscosity to provide superior flow, thermal removal characteristics and economy.
The reason why I note this is in response to the confusion and commentary that it is all about economy, as if protection of the engine was automatically compromised. It isn't. MB doesn't want you using some [censored] multigrade. That is true. But when the oil with correct robustness in terms of viscosity under stress is applied, it becomes a non issue.
40wt was specified 30 years ago (and 10w-40 was not allowed) because oils were far less robust and less spec controlled. As these ratings came forth to judge the duty and use characteristic more clearly, the opportunity to use alternate, superior fluids was expanded because it started to be more foolproof and a spec could draw the right product without issue. When MB expanded the 10w-30 temp range, the only thing that happened was that a high HT/HS performance fluid was able to be specced by invoking the G5 part. All of a sudden you could have a superior foil in use, with a lower viscosity at operatic temps because you knew the quality and metrics of the oil being used. It wasn't just someone dumping 10w-30 havoline in there for fun.
So with the enhanced specifications have come easily recognizable highly robust oils that can do the same or better than a straight weigh oil, while offering superior starting properties when "cold", I.e. Not operating temp.
So youre really looking at 15w-40 vs 20w-50... Personally, if the engine uses no oil and has no leaks, Id run delo or rotella 5w-40 syn in there. Id also consider M1 HM 10w-40 which is a stout A3 oil. Im not going to put every last demand on having a super thin oil at startup, but I like the concept. And, unless the conditions are VERY hot, I just dont see the need for the heavier viscosities when MB says that lighter oils with known robustness (e.g. ACEA A3) are suitable. Id be happy with a good 40wt unless you live in death valley.
Now, whether you want to go for a 5w-40, 10w-40 or a 15w-40 is another story...
