Some food for thought. I drained and sent out an oil sample for lab testing to see if I should be concerned about rebuilding my drive since I saw some fine metal in the oil. Apparently the metal content was quite low for the sample and the "machine condition" passed with no problem. However... I ran this drive for ~40 hrs since the last oil change. Volvo recommends 200hr drive oil change intervals for the SP aquamatic. The "lubricant condition" however was "marginal". The measured viscosity level was very low. Possible causes were from thermal cracking of oil molecules, or shear thinning of viscosity improvers... The oil used was Castrol GTX 10w40. I know nothing destroys oil quicker than gear sets hence why motorcycles demand specialty oils with shared motor/transmission oil. But didnt expect to eat through it so quick, especially since Volvo recommends "same as engine" oil. So for you guys running older SP drives with motor oil maybe consider running a higher quality one. I figured oil was oil but I was completely wrong... Just something to share.
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