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Hi there friends, I promised I ran a few searches for the answer but couldn't find it.

My 77 CB750 is out of oil, just about. The previous owner installed a (poor) aftermarket tank that doesn't have a vent, just a magnetic cap which acts as a vent. I understand on these bikes the vent line connects to the nipple on the back of the motor to reduce wasting oil, which with this tank is obviously impossible, so the pressure in the tank caused by heat and the scavenger pump causes oil to slowly leak from the cap. Not to worry, I have Cognito Moto making me a tank, which is designed specifically for this bike.

It won't be here for a week or two however, and I want to keep riding the bike so I can keep tuning it. My question is as follows: can I just shove some more 10w40 mineral in the tank and keep riding for the time being, or do I need to carry out a proper oil change before I get back on it? I'm concerned the previous owner may have used synthetic and I don't want to cause any problems. I have a full oil change kit in my workshop ready for the new tank, I'm just reluctant to do it twice.