Quick q:
I am a new owner of a 2/3 streetported engine with non-seq turbos - I'm about ready to change the oil completely and am leaning towards royal purple, what kind should I buy:
5W20, 5W30, 10W30, 10W40, 15W40, 20W50?
I live in Pittsburgh, PA also so I'm dealing with all sorts of weather conditions hot - cold and everything inbetween.

 

winter = 10w30
summer + 20w50
15w40 = diesel oil

 

I use a semi-synthetic by Schaeffer's that while doing oil-analysis on my Mercedes AMG-powered wagon and Mercedes 500E, showed excellent results compared to running Mobil1 15w-50 or Mobil Delvac 1.

A quality semi-synthetic can outperform lubrication qualities of a synthetic, but not the extended drain interval.

Since I change the oil and filter every 3K miles, I don't want the added expense of a 100% synthetic.

Here are my results from the Mercedes, (RX7 coming soon):

http://theoildrop.server101.com/ubb/...=001583#000000

http://theoildrop.server101.com/ubb/...=001584#000000

This website and forum is passionate about oils, they even have "virgin" and "used" oil analysis samples:

http://www.bobistheoilguy.com/
http://theoildrop.server101.com/cgi/ultimatebb.cgi

 

Here's an excerpt of the C36's oil sample:

1988 Mercedes 300TE
32,000 miles on 1995 AMG M104 DOHC Inline-6 engine
3,500 miles on sample (per Schaeffer, no TBN)
Recent head & valve-job 3,500 miles ago.

Wear Metals (PPM)
Copper: 7
Iron: 24
Chromium: 2
Aluminum: 7
Lead: 4

Additives (PPM)
Moly: 147
Phosphorus: 1929
Zinc: 2048
Magnesium: 39
Calcium: 5380
Sodium: 0
Postassium: 0

Contaminents:
Silicon: 11

Oil Properties:
Viscosity: 15.22
SAE: 40W
Sulfur: 16
Oxidation: 4
Nitration: 7

Contamination & Additive depletion levels normal.
Increase oil drain interval to 5,500 miles.