Tiag knows his ish, but I use 20w50 almost exclusively (used 0w20 in my Mustang GT that was purchased new) in my bimmers... I've never had any problems over my fleet of 10 over 10 years in the Texas heat. YMMV though of course. That said, in the X5 and X3 I just got some Valvoline Full synth 10w30 on sale for a ludicrous price so that went into the X5 and X3 recently.

Your knocking noise, unfortunately, sounds like a classic case of chain guides/rails failing. Check it out before continuing to drive it. Drop the lower pan and have a gander to see if you have bits of guides/plastic in there.

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2005 X5 4.4i Build 04/05 Maintenance/Build Log
Nav, Pano, Sport (Purchased 06/14 w/ 109,000 miles) (Sold 8/15 w/121,000 miles)

2006 X5 4.8is Build 11/05 Maintenance/Build Log
Nav, DSP, Pano, Running Boards, OEM Tow Hitch, Cold Weather Pckg (Purchased 08/15 w/ 90,500 miles)

2010 X5 35d Build 02/10
Nav, HiFi, 6 DVD, Sports Pckg, Cold Weather Pckg, HUD, CAS, Running Boards, Leather Dash, PDC, Pano (Purchased 03/17 w/ 136,120 miles)