Last season our league and the house agreed on having two different patterns put out. One for the first half and the other for the second half. In reality, we had many more patterns than that.
The house has three lane machines, each about a decade apart in age. When the primary one, the newest, would break down, they had to use the next oldest one but that one could not put down the same patterns as the newest so the pattern would be different (it was programmed with an older pattern from years ago). When that one broke down, they had to use the oldest one which was a much older pad type and it had a different pattern pre-installed. Not to mention the couple of times that the mechanic accidentally hit the wrong button and put a different pattern out on the primary machine (which can hold dozens of patterns).
In the end, it doesn't really matter as you are bowling against a team bowling on the same pattern. The only times it would be of concern would be if a different pattern was put down for a team doing a pre-bowl or a make-up so the two teams bowled on different patterns, one of which would/could be an advantage to one team.
As to the volume (thickness) of the oil varying - there are a lot of other variables such as weather or humidity that can change each week making a difference too.