[Blue Bowtie]

Pennzoil/Quaker State (same company) have historically (past evidence - They may have recently changed) used the lowest quality base stocks for blending consumer engine oils. That means that in order to meet API requirements, they have used fortifiers extensively. The polymers used to fortify oil typically fail early in their life cycle, burning and turning to sludge. A good oil filter will be able to trap some of that, but that leaves a lot less filter to trap other particulates.

Beyond the problem with sludge in the filter and the rest of the sludge in the engine, the rapid loss of polymers means the oil quickly reverts to its base form. The base oil that would once maintain viscosity at 300º with the aid of polymers becomes thinner than water, and provides very little lubricant film strength.

Engine oils are typically 75-85% base stock, with the remaining volume being comprised of additives. Because of this, base stock is critical to the performance of the final product. Poor quality base oils require more additive, and the additives frequently become contaminants as they are used. Beyond the tendency of the oil to fail, the volume of usable oil decreases as the filter and engine become clogged with sludge. Better oils, even mineral oils using better base stocks, do not suffer that tendency to anywhere near the same degree. Group IV synthetics do not suffer it at all.

If you use low-grade oil and change every 1,500-2,000 miles, you might just get by without serious engine wear and sludge. That's a gamble, but in the long run, you've saved nothing over using a superior lubricant changed one-third as often. You can take the big step to Group IV synthetics and use it confidently for up to 10,000 miles, more if you care to sample and analyze it.