If this helps as its a real can of worms topic but to keep it simple, the 2 numbers for oil are measurements of thickness.
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The closer to 0, the thinner the oil, the higher the number the thicker the oil. Old engines prefer thicker oil and it doesn't leak so freely (if it leaks).
The W stands for winter (cold). So 10w (cold) is thinner than 20w(cold), and 40(hot) is thinner than 50(hot). In the UK 15W and 40 is ok. (so good down to -15 and up to 40deg outside)
So even though OD is right, not good practice to mix, in truth some oil is better than none and 15W40 isn't too far off 20W50.
Regular changes are good so if you've just bought a shed load of 20W50 for your next service or if its all you've got then it will be ok.
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Don't mix synthetics and semi and mineral oil though. Only 1 type at a time in your engine.
Make sure your rocker covers are super snug and tight. Only stick sealant on the cover, not the rocker side.
I know people who use synthetic 5W 30 without problems and.. well, I'll let you know how I get on