"Can I run 5W-30, or does my E36 need something thicker?" and "what's the best oil for an E36 M3?" are the same question at heart: which viscosity. The safe answer for the whole range is a full-synthetic 5W-40 meeting BMW Longlife-01 (LL-01) / ACEA A3/B4 — but 5W-30 is fine in the right conditions, "thicker" helps only in specific cases, and the M3 does not need the 10W-60 people assume
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