First, the assumption for me is that the engine has gone at least 25,000 miles. Based upon my experience that is where the silicon gaskets/seals have had time to break in. Then, I like to see under 15 ppm at my interval which is 7500 in most cases. This equates to about 2 ppm per 1000 miles. However, if the iron numbers are low and have not spiked and the other wear numbers have not spiked the silicon is probably immaterial to wear and it is coming from other then dirt. My 92 Camry has 155,000 miles now and over a dozen lab samples at 7500 mile intervals. The silicon has consistently been in the teens and at times considerably higher. You really have got to look at the trend and miles etc. etc. there is no magic number. Like the copper number on GM engines, kind of relative. I would normally freak at 90 ppm of copper if I did not know that this is common for the engine in question. Maybe I live in a dirty climate but most of the time my silicon is in the hgiher teens at 7500 miles. Most labs, I believe, cut off at about 30-35 ppm of silicon regardless of mileage on the oil and then they flag it as high.