My xr3i sounds fine when cold but as soon as it gets warm, when reving it hard it sounds rough, really dry. Above 3k you can hear the tappets! I am using magnatec 10W40 semi synthetic oil, i am thinking thicker oil may help it! THe cam and tapppets are relatively new, 4k and it was run in properly. Sounds to me that the oil is getting too thin! What do you lot think
I have done an oil pressure check a little while ago and it was a fraction above what the Haynes manual says so @ 2k around 40psi

 

Either your oil pumps knackered or (dare i say) your new cam and lifters is