Oil changes every 10K miles - really?

Factual Questions
21

I’ve got a classic Ford truck. Hi mileage. Burns a little oil.

I thought experts generally frown upon both flushing an old engine and running synthetic through an old engine that’s never run synthetic before, claiming the latter causes leakage through the seals.

22
  1. Increased levels of detergent, anti-wear and anti-oxidant additives.
  2. Small quantities of esters for cleaning. Some esters can be highly effective at getting the gunk out of your engine, unsticking stuck reings, etc. Esters are technically “Group V synthetics”, and I’ve seen a few high-mileage formulation that used something like 15% of this synthetic oil base stock in their blend. [1]

[Note: entire MrSlant quote above; I couldn’t format his list in the quote box?]
Some high mileage oils contain additives that cause seals to swell, the idea being that seals tend to dry out and contract slightly with age. If the additves work, it might be more likely to help with minor leaks, such as someone burning oil due to leaky valve stem seals, but might not help someone losing oil through a major rear main seal leak. Obviously seals vary quite a bit in composition and the degree to which they will swell when exposed to seal swelling agents. Since the price increase is not much, it might not hurt to try these oils on a higher mileage engine, but I wouldn’t expect miracles.

23

As I understand it, the claims about synthetics causing seals to leak, especially in older cars, were completely true.
The only thing is, they were true back when Mobil1 was first introduced and was made of pure Group IV synthetic plus a small amount of conventional oil used purely as an additive carrier.
Once ExxonMobil figured out that they had this problem, they started mixing in some Group V products to counteract the seal-shrinking effects of the Group IV.

Regarding not adding extra cleaning agents to a high-mileage engine, there are a number of issues. If you have “false seals” in your engine, that is, your engine doesn’t leak only because there’s nothing but sludged motor oil and carbon build-up plugging some of the gaps, a good flush can cause leaks.
The scariest issue with some cleaning agents is breaking off big deposits of various kinds and having them wind up getting caught in engine components with low tolerances.
This kind of cleaning doesn’t happen with motor oils or sane engine flushes. What motor oils and the Auto-Rx product I mentioned before do is clean gradually over the course of hundreds of miles.
Now there are some flushes that might be a little bit risky. Some products basically have you completely fill your crankcase with a powerful solvent and then idle the car for five minutes. That’s a good way to cause quick trouble if you have a '91 Chevy Suburban with 350,000 miles on the original engine and no previous deep engine cleaning.