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Many of us did effectively the same thing years ago when we bored B18 blocks for B20 pistons. IPD used to sell a kit for that, with no mention of changing the head from B18 to b20. I had one in a 544 for years that ran great with a C cam. The main advantage of going to the B20 head is to improve breathing (power,) compression shouldn't be an issue unless it's the rare high-compression B18 head. If it runs fine, I wouldn't worry.
A compression test is most useful for comparing results between cylinders to determine engine wear. The oil sounds like a crankcase ventilation issue as George mentions.
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67 144, 85 740T, 86 740T, 91 945SE, BMW R69S, R60/2
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