5w is 5 weight, 10w is 10 weight, 20w is 20 weight in cold temperature. The oil does not thicken . It just protects to the number as 20-30-40-50. 5w is good for a fresh start up. Increase the weight the more miles it gets. My motor new cold would peg the gauge with 5 weight. Then go to around 50- 60 when hot. After some years I was running 50 weight race oil. Seen some pretty high RPMs above 9000. for 6 years. Bearings were like new. rings were waisted. Rear of the thrust bearing was wore but not bad. The worst thing in the motor was the intake push rods were gulled. They were oil through junk. I now went to good thin heavy wall solid balls from smith bro's. I always oiled through the rockers . so the oil through push rods were not for me.
Indexing and running a ball mill up the stud for the head is necessary.
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