Butadiene rubber surged 3.71% to ¥15,380/t—the strongest daily gain across all commodities—outpacing natural rubber (+1.44% at ¥19,395/t) and asphalt (+0.16%). This divergence is not noise: it reflects structural substitution in lubricant blending. With domestic refinery turnarounds concentrated in late September (Sinopec Maoming and Zhenhai bases), supply of Group I/II base oils has tightened. Lubricant blenders are accelerating use of butadiene rubber as a viscosity-index improver to replace part of PAO or high-VI mineral oils. The butadiene–natural rubber spread has widened to ¥3,985/t, approaching the 2025 average of ¥4,295/t (industry benchmark), confirming economic viability has reached a tipping point. Concurrently, fuel oil +3.33% to ¥4,253/t sets a hard floor on base oil cracking costs: current residue–fuel oil spread implies Group I base oil gross margin at ¥1,820/t—¥410/t lower than August’s average—suppressing new run-rate incentives. USD/CNY +0.15% to 6.7108 raises landed-cost pricing but also widens arbitrage: SK (Korea) and Reliance (India) spot offers into East China now trade at ¥280–¥320/t discount, translating to landed prices of ¥6,150–¥6,190/t—3.3%–5.0% below domestic Group I spot at ¥6,470/t. SNSUC Research Institute data shows East China bonded warehouse base oil stocks fell 4.2% w/w in Week 3 of September, while import customs clearance volume rose 17.6% YoY—confirming substitution is operational, not theoretical. Key risk: if Brent holds below $95/bbl, refiners may shorten turnarounds, capping spot premiums; conversely, if WTI breaks $90/bbl and triggers US shale cuts, distillate tightness will lift base oil cracking margins. For SNSUC clients, this is the final window to fix October–November import cargo pricing—lock at fuel oil futures + ¥1,200/t, and hedge USD/CNY exposure.