Amsoil Signature Series is the pick for outright engine protection. It
produced the smallest wear scar at 5.22 mm, lost the least oil to heat at 4.14
g after two hours at 400°F, and the lab analysis explains both results — it
carries the heaviest anti-wear additive package here at 2,129 ppm. The price
is the problem: $58 a gallon, four times what the cheapest oil costs, and it
was the worst performer by a wide margin at -40°F. Schaeffer’s at $42 gets
most of the way there for less, with a 5.27 mm wear scar, 2,042 ppm of
anti-wear additives, and the largest detergent and dispersant package in the
group at 2,141 ppm.
Rotella T6 is the value pick at $23. It never won a test outright, but it was
quickest of the four thinner oils at -40°F both fresh and after heat exposure,
at 134.2 and 142.8 seconds, and its 5.63 mm wear scar and 6.64 g of
evaporative loss put it squarely mid-pack against oils costing twice as much.
A heavier anti-wear additive package would make it the obvious default. If
cold starting is the priority instead, Hot Shot’s Blue Diamond at $55 is in a
class of its own — 95.4 seconds at -40°F against 134.2 for the next-quickest
oil, and still fastest at almost 108 seconds after the bake — though its
anti-wear additive content is on the lighter side for the money.
The cautionary result is Mobil Delvac at $14. Its 1,451 ppm of anti-wear
additives is well below every other oil here, and it finished last on both the
lubricity rig at 5.94 mm and the evaporation test at 6.75 g. Royal Purple at
$30 is a mixed case: excellent evaporation resistance at 5.25 g and a solid
5.6 mm wear scar, but the heat exposure cost it noticeably more cold-flow
speed than it cost the others, and its anti-wear content sits below the two
leaders. The broader lesson is that no single oil here wins on both axes —
thick oils protect under load, thin oils protect at start-up, and the right
choice depends on which of those an engine sees more of.